In order to have a large flying hexapod, you might want to consider adding:
- Flaps between limbs
- A habitat near some form of updraft - ocean winds, volcanic wind, or some manufactured thing.
- Honeycombed bones; an organ to create hydrogen
- An incredibly large diet; perhaps hibernation when this isn't possible
Depending on how dense the atmosphere is, flying could end up being much more similar to swimming and accordingly, birds could become more similar to fish. Therefore they could grow to be large as long as they had light, maybe hollow bones and incredibly strong muscles.
I’m not sure how dense the atmosphere you’re working with is, but if it becomes extremely thick the creatures that could fly could begin to deviate from what a traditional bird looks like: namely, they could become similar to a family of insects called the mymaridae, the species which comprise it which don’t have traditional wings but rather have limb-like extensions covered in bristles, and using that they can fly through the air as if they’re swimming: the Arescon and Myanmymar are examples of this.
Less dense atmospheres
If the atmosphere on your world isn’t that dense, then a solution would have to be worked out some other way: trying to figure out how to minimise the amount of lift that your aerial predator needs.
Since generally the larger an object is the more lift it needs - its mass growing as its length cubed and the surface area as its length squared - and therefore needs bigger wings in proportion to its body as it grows.
Of course, as the creature needs larger wings, the material that makes up the wing must also accordingly be stronger lest the wing collapse under its own weight.
As an example, Pterosaurs on Earth in prehistoric times were shaped like a kite: a relatively small main body, flat for aerodynamic reasons, with flaps of skin between limbs to increase surface area, and ginormous wings. Relatively recent hypotheses also suggest that these dinosaurs would live near coastal cliffs and jump off of these, relying on the ocean gusts to get them airborne - at which point they would mainly glide.
Following that, any creature with relatively small wings and large body mass would never be able to fly, and flapping their wings wouldn’t work.
Other misc issues; food consumption and internal structure
This issue was discussed and solved on an old Discovery Channel show called “Dragons: Myth Made Real”; in order to come up with a way for dragons to fly they determined that they must possess light yet durable honeycombed bones that were light but durable and an inflatable bladder that could be filled with hydrogen produced with bacteria in their stomachs (or perhaps they could have a separate organ specifically dedicated to the production and excretion of this hydrogen - or perhaps the organ could pump hydrogen into the honeycombed bones? The choices are endless… )
Further, birds must eat about ¼ to ½ of their weight in food each day; this assumes an average weight-to-wingspan ratio. However, as discussed above, this ratio must increase as the size and therefore mass does - I wouldn’t be surprised if your aerial predator had to consume upwards of 3/2 times its body mass each day if it flew all the time (and didn’t glide: if it were primarily a glider it might need to consume less). Therefore, in times when this mightn’t be feasible perhaps it would sleep - maybe even hibernating in winter when other things hibernate because it would be harder to find food.
Behaviour - I know you didn’t ask for this but I couldn’t help myself
If these predators act in packs and don’t live isolated lives (this might make more sense than an isolated life given how much food they need to eat - and also it would make them more of a formidable enemy) then perhaps it would function much like a pride of lions; the leader doesn’t need to expend energy hunting and instead the females and young members serve the food to the rest of these creatures.
On the other hand, the pack (maybe a phalanx? See https://www.backyardchirper.com/blog/collective-nouns-for-groups-of-various-birds/ for more) would be hierarchical in that those with the most kills/whatever would be seen as higher-up and therefore a more competitive nature would be uncovered, perhaps leading to some members sabotaging others.
Alternatively, if these creatures live isolated lives then they could perhaps be found dying all over the place, desperate - especially in winter months. This desperation could drive them to cruelty and dive-bombing even the largest groups of creatures that they would have no hope of overcoming because they are so close to death. Or they could be ruthless and complete in the annihilation of their prey and kill any other competition in their area.
Pennycuick, C. J. Modelling the Flying Bird. 2008.
What is the maximum size of a flying creature?