Being generous, at most something on the scale of WW1
Whilst other answers address some important considerations with geography, there is something that is being heavily overlooked. Logistics
Perhaps the single greatest thing that contributed to the scale and brutality of both World Wars wasn't any weapon, rather it was industry.
The industrial revolution brought with it many changes that greatly changed warfare in general. On the more technical level, more advanced metallurgy, higher quality smaller parts that could be made on mass. Increased production of traditionally expensive weaponry, better ships/hulls, better clothing etc.
Scale and quality greatly improved. Whilst you have no gunpowder in your setting, you still have to answer to the scale question.
The industrial revolution not only improved quality but churned out numerous pieces of items at a much larger and faster rate than ever before. The advent of better machinery and tools had also meant that labor and manpower hour for production were sharply cut down. Equally important, it also standardized the quality of equipment.
The mass production of military equipment had meant that warfare took a more deadly and wider scope. One of the first modern wars, the American Civil War, was particularly brutal due to better weaponry that was produced on mass. The quality of a musket or rifle wasn't dependent on the skills of a certain blacksmith. Rather the tools and quality control of a factory. This means that both sides, but especially the North, could field large armies very quickly. The training needed to fire off a musket or rifle was significantly smaller than that of a spear or sword. It takes less time to make a soldier proficient with a firearm compared to that of knights, bowmen, calvary, siege engine crews etc.
However, you want something on the scale of World War 1 at least. By now the industrial revolution has more than matured and left its mark. The scale of industry is far larger than compared to the mid 1800s. Rifles, artillery, bullets and artillery shells are produced at an incredibly large level but in a short time as well. HMS Dreadnought set off an arms race for the ocean as engine, artillery, and metal technology saw massive gains compared to the days of early ironclads. This technology percolated throughout the rest of the military and rest of society.
Staying with the first World War, some of the things that made it so deadly and large in scale were:
- Access to firearms that can be quickly and easily trained with.
Deadlier ammunition that could be created on mass. Conscript armies
that are of higher quality than any peasant conscript army with the
ability to inflict far more damage.
- Large scale production of artillery and ammunition
- Better engines
- Armored ocean going vessels capable of holding significant weight and
- Transportation ships that could haul far more in goods and much
faster. So much so, that the United States and Canada could heavily
support the United Kingdom in both wars, as well as the Soviets
during the WW2
- Better clothing for adverse environments
- Factories with the ability to change production and address
deficiencies in certain areas (see Shell Crisis of 1915).
- Long and better range communications. Radios changed warfare greatly.
- Better medicine as well as hospitals. Combat medics and surgery for
injured soldiers were far more intensive than compared to previous
- Increased attention to media, literacy and education. With the power
of nation states having increased so greatly by now, the idea of a
national spirit/unity cannot be understated. Technology, improved
literacy and propaganda allowed people to reach a much larger
- Railways/trains. It cannot be understated how important rail was for
both World Wars. Not only did they move soldiers to a frontline in
the event of mobilization, but they greatly aided in transportation
of equipment from factories to the front line.
Plus, many more.
However, your fantasy setting has some roadblocks in the way of getting a true WW1 or WW2 scale. Even with a light industrial revolution.
For example, the lack of ocean shipping means that logistical support over ocean or force projection is basically impossible across water bodies. You simply don't have the sea faring capabilities to sustain an invasion across an ocean or a large water body. You won't be doing any island hopping, massive multi front amphibious assaults (Operation Overlord + Operation Dragoon). Supporting allied factions across a body of water in a meaningful way on the likes of WW1 is going to be very hard if not impossible. Whilst a land army can march (Grant+Sherman and the Romans for example), they're going to either have to rely on the land or strain their logistics heavily. If you don't have food preservation technique on an industrial scale, you have an even larger problem (especially for war elephants). Your army runs the risk of starving out if any defending and retreating faction employs a scorched earth policy if your invading force scavenges for food.
Combat Losses- Replacing combat losses is going to be much harder than say WW1. Even if we are being generous and mass arming your soldiers with crossbows capable of piercing plate armor reliably and spears, both still require a decent amount of training to become battlefield proficient. Every soldier that you lose, especially veterans, is going to heavily impact your army. Reinforcements are going to take a lot longer to come up to speed and perform cohesively. A faction runs the risk of being steam rolled should they sustain either mass or even key combat losses. You have no artillery to equalize or force multiply. You can't just spend a few weeks training a conscript how to use a crossbow or spear and then ship them off to the equivalent of Gallipoli or Verdun.
The mass casualty combat events on a scale of WW1, even worse WW2, isn't going to be sustainable for both your army and your industrial system.
Industrial production- The lack of things like gunpowder and advanced industrial mining means that getting ores and metal is going to be a costly and time consuming process. Especially on short notice. The machinery brought on by the industrial revolution by the turn of the 20th century greatly allowed nations to quickly mine and produce goods and equipment. Complex networks existed to supply factories.
Fortifications- Forts are going to prove to be far more deadly than that of WW2. WW2 introduced the concept of large-scale maneuver warfare. Forts could be bypassed, left to be dealt with at a later day. While both the Allied and Axis forces did fight battles over forts, they didn't pose the same power as pre cannon days. The invention of bombardment types of weaponry gave rise to an entirely different type of siege warfare that your forces don't have access to. Forts and trench lines in WW1 proved to be a difficult task to break through, tanks helped alleviate some of the issues. Heavy artillery helped as well. However, your fantasy army will have to deal with them. Starving them out takes time and requires a contingent of soldiers to watch over them. Taking a fort by force is going to be costly, even if you have a lot of siege weapons. This is going to break up your force concentration ability. Sieges also take a toll on both factions' logistics. Whilst this may appear to make a static frontline like WW1, you are going to lack the force maneuvering that WW2 saw. Theater wide scale will be brought down; however, you will now have an established frontline.
Transportation- The lack of trains and rails means that you can't transport troops and equipment quickly across long distances. You also won't be able to quickly redirect reinforcements or resupply a frontline. Every horse, or pack animal, that you lose, is significantly hurting your ability to wage a world war. Living animals take time to grow. Yes, ancient and pre modern armies marched and supplied themselves on foot. However, their scope of conflict didn't have the same breath/width of combat that WW1 and WW2 experienced at the same time across fronts.
Electric Communications- Whilst large land campaigns have been conducted without the use of electronic communications, both WW1 and WW2 heavily benefited from things like wire and radio communications. Whilst soldiers in WW1 didn't have personnel radios like now. Larger armies, generals and nations had the ability to quickly communicate with each other. For your fantasy setting, your central headquarters is going to have some issues in getting information about the frontline, who is maneuvering where, and creating a composite image of enemy force concentrations and dispersion. Even if you leveraged your magic system for tradecraft, it still is going to take time for that information to reach someone, and for an action to be taken in response.
So how do you get something near World War 1?
As others have stated, geography. Each faction has to be close enough in that they can quickly bring reinforcements and equipment to the frontline. With little space in between, invasions are more easily to be sustained. Your light industrial revolution has also caused the explosion of maintained roads. These roads will be the closest thing to rail you can get. Because of the close proximity of sorts, a cold war of sorts has been developing. So, your standing army also has significant reserves back home. Even better if something like a national guard or home guard exists. World War 1 was mainly fought in Europe and adjacent regions. There was no island hoping, daylight bombing raids on the scale of the Second World War, maneuver warfare using mechanized forces like Operation Cobra or Arracourt. After the initial German breakthrough buckled and slowed by Paris, trench warfare and stalemates started to become more of the norm. You can replicate this through the use of fortification lines with numerous forts. Each fort should have enough resources to sustain a siege and supply and reinforce another fort in danger.
Lastly some perspective. The Thirty Year War claimed some 4.5-8 million soldiers. Germany fielded an army around the size of 3 million in just the invasion of the Soviet Union during Operation Barbarossa. The Soviets would lose about 4.5 million soldiers during said invasion.