Your biggest problem is that

###Smaller pillars will collapse under their own weight###

Assume your pillars are made from the surrounding rock, and that for reasons of practicality that rock is sandstone. (Strong in compression, relatively easy to quarry.) In the real world, we have sandstone pillars such as the Old Man of Hoy. That's only 137m, but it's about 10-15m wide. It is one of the largest free-standing pillars in the world, and probably the largest with near-vertical sides. Interestingly, that gives it similar dimensions to the tower of Ulm Minster. Both of these only need to support their own weight, not the weight of any ceiling above them, so any supporting pillar made of stone would necessarily be larger than this. And both these are still only a third as high as your target. Width tends to increase by the square of height, so you could reckon on your pillars being 9 times wider. And they still can't take any extra load yet.

Based on this, we simply can't escape having enormous pillars if we're leaving them in place from the quarried rock. Even reinforced concrete won't give us substantially different performance to solid stone.

The only answer then is

###Make the pillars out of something other than rock###

Skyscrapers such as the Burj Khalifa certainly exceed this height, and the actual top floor of the Burj Khalifa is about the height of your ceiling. (The rest is just a spire.) Skyscrapers use a steel framework, so your roof pillars could certainly use a similar principle. You could even construct them as skyscrapers, making the pillars somewhat wider but using them as residential/office space to increase the living space available. This would be a logical use of the volume of the cavern, instead of building structures from the cavern floor. Alternatively the pillars could be made smaller than a skyscraper width if all you wanted them for was structural strength.

Conceptually, the volume of the pillars could be reduced further by using more exotic construction materials such as Kevlar or carbon nanotubes. (Maybe the Lizard Empire have technology which we don't have yet.)

As an aside, I find the idea that the population would just live on the cavern floor is boring, unimaginative and frankly also naive and unrealistic. I wouldn't buy into that as a worldbuilding concept, unless we assume a "City of Ember" scenario where the whole place has reverted to semi-barbarism. Living space is about surface area, and there's a lot of surface area up those pillars and inside the pillars. Also up the cavern walls (like humans, lizards will not respond well to fully-enclosed apartments, but apartments looking out over the cavern would be cool and even desirable), and even in the cavern roof. And with three-dimensional living spaces and more advanced technology, we could also assume three-dimensional transport systems, with high-speed cable-cars running between pillars at different heights along a web of transit cables. Build a world which fits the technology and space. Show us something new. Don't just give us yet another copy of 1930s Manhattan underground.

Your biggest problem is that

Smaller pillars will collapse under their own weight

Assume your pillars are made from the surrounding rock, and that for reasons of practicality that rock is sandstone. (Strong in compression, relatively easy to quarry.) In the real world, we have sandstone pillars such as the Old Man of Hoy. That's only 137m, but it's about 10-15m wide. It is one of the largest free-standing pillars in the world, and probably the largest with near-vertical sides. Interestingly, that gives it similar dimensions to the tower of Ulm Minster. Both of these only need to support their own weight, not the weight of any ceiling above them, so any supporting pillar made of stone would necessarily be larger than this. And both these are still only a third as high as your target. Width tends to increase by the square of height, so you could reckon on your pillars being 9 times wider. And they still can't take any extra load yet.

Based on this, we simply can't escape having enormous pillars if we're leaving them in place from the quarried rock. Even reinforced concrete won't give us substantially different performance to solid stone.

The only answer then is

Make the pillars out of something other than rock

Skyscrapers such as the Burj Khalifa certainly exceed this height, and the actual top floor of the Burj Khalifa is about the height of your ceiling. (The rest is just a spire.) Skyscrapers use a steel framework, so your roof pillars could certainly use a similar principle. You could even construct them as skyscrapers, making the pillars somewhat wider but using them as residential/office space to increase the living space available. This would be a logical use of the volume of the cavern, instead of building structures from the cavern floor. Alternatively the pillars could be made smaller than a skyscraper width if all you wanted them for was structural strength.

Conceptually, the volume of the pillars could be reduced further by using more exotic construction materials such as Kevlar or carbon nanotubes. (Maybe the Lizard Empire have technology which we don't have yet.)

As an aside, I find the idea that the population would just live on the cavern floor is boring, unimaginative and frankly also naive and unrealistic. I wouldn't buy into that as a worldbuilding concept, unless we assume a "City of Ember" scenario where the whole place has reverted to semi-barbarism. Living space is about surface area, and there's a lot of surface area up those pillars and inside the pillars. Also up the cavern walls (like humans, lizards will not respond well to fully-enclosed apartments, but apartments looking out over the cavern would be cool and even desirable), and even in the cavern roof. And with three-dimensional living spaces and more advanced technology, we could also assume three-dimensional transport systems, with high-speed cable-cars running between pillars at different heights along a web of transit cables. Build a world which fits the technology and space. Show us something new. Don't just give us yet another copy of 1930s Manhattan underground.

Your biggest problem is that

###Smaller pillars will collapse under their own weight###

Assume your pillars are made from the surrounding rock, and that for reasons of practicality that rock is sandstone. (Strong in compression, relatively easy to quarry.) In the real world, we have sandstone pillars such as the Old Man of Hoy. That's only 137m, but it's about 10-15m wide. It is one of the largest free-standing pillars in the world, and probably the largest with near-vertical sides. Interestingly, that gives it similar dimensions to the tower of Ulm Minster. Both of these only need to support their own weight, not the weight of any ceiling above them, so any supporting pillar made of stone would necessarily be larger than this. And both these are still only a third as high as your target. Width tends to increase by the square of height, so you could reckon on your pillars being 9 times wider. And they still can't take any extra load yet.

Based on this, we simply can't escape having enormous pillars if we're leaving them in place from the quarried rock. Even reinforced concrete won't give us substantially different performance to solid stone.

The only answer then is

###Make the pillars out of something other than rock###

Skyscrapers such as the Burj Khalifa certainly exceed this height, and the actual top floor of the Burj Khalifa is about the height of your ceiling. (The rest is just a spire.) Skyscrapers use a steel framework, so your roof pillars could certainly use a similar principle. You could even construct them as skyscrapers, making the pillars somewhat wider but using them as residential/office space to increase the living space available. This would be a logical use of the volume of the cavern, instead of building structures from the cavern floor. Alternatively the pillars could be made smaller than a skyscraper width if all you wanted them for was structural strength.

Conceptually, the volume of the pillars could be reduced further by using more exotic construction materials such as Kevlar or carbon nanotubes. (Maybe the Lizard Empire have technology which we don't have yet.)

Your biggest problem is that

###Smaller pillars will collapse under their own weight###

Assume your pillars are made from the surrounding rock, and that for reasons of practicality that rock is sandstone. (Strong in compression, relatively easy to quarry.) In the real world, we have sandstone pillars such as the Old Man of Hoy. That's only 137m, but it's about 10-15m wide. It is one of the largest free-standing pillars in the world, and probably the largest with near-vertical sides. Interestingly, that gives it similar dimensions to the tower of Ulm Minster. Both of these only need to support their own weight, not the weight of any ceiling above them, so any supporting pillar made of stone would necessarily be larger than this. And both these are still only a third as high as your target. Width tends to increase by the square of height, so you could reckon on your pillars being 9 times wider. And they still can't take any extra load yet.

Based on this, we simply can't escape having enormous pillars if we're leaving them in place from the quarried rock. Even reinforced concrete won't give us substantially different performance to solid stone.

The only answer then is

###Make the pillars out of something other than rock###

Skyscrapers such as the Burj Khalifa certainly exceed this height, and the actual top floor of the Burj Khalifa is about the height of your ceiling. (The rest is just a spire.) Skyscrapers use a steel framework, so your roof pillars could certainly use a similar principle. You could even construct them as skyscrapers, making the pillars somewhat wider but using them as residential/office space to increase the living space available. This would be a logical use of the volume of the cavern, instead of building structures from the cavern floor. Alternatively the pillars could be made smaller than a skyscraper width if all you wanted them for was structural strength.

Conceptually, the volume of the pillars could be reduced further by using more exotic construction materials such as Kevlar or carbon nanotubes. (Maybe the Lizard Empire have technology which we don't have yet.)

As an aside, I find the idea that the population would just live on the cavern floor is boring, unimaginative and frankly also naive and unrealistic. I wouldn't buy into that as a worldbuilding concept, unless we assume a "City of Ember" scenario where the whole place has reverted to semi-barbarism. Living space is about surface area, and there's a lot of surface area up those pillars and inside the pillars. Also up the cavern walls (like humans, lizards will not respond well to fully-enclosed apartments, but apartments looking out over the cavern would be cool and even desirable), and even in the cavern roof. And with three-dimensional living spaces and more advanced technology, we could also assume three-dimensional transport systems, with high-speed cable-cars running between pillars at different heights along a web of transit cables. Build a world which fits the technology and space. Show us something new. Don't just give us yet another copy of 1930s Manhattan underground.