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I experimented further with the battle arena sizes.
As it turns out, the range of a direct arrow shot is equal to 60% of the arena's width, regardless of its length.
For example, with a width of 10 (standard), it will be 6.
And with a width of 14 (to accommodate all 7 large units), it will be 8.
However, the camera's viewing distance over the battlefield depends on its length, i.e., the distance between the players' sides. The longer the arena, the higher the camera, and the smaller the units appear.
Added 1 hour 49 minutes ago
In general, 14x14 is the optimal size, but the arenas themselves will need to be reworked (not all of them are visually suitable).
I played around with the speeds - +1 for everyone, including flying units, is the optimal solution.
Small flying units will then have a speed of 7. Archers will not be able to approach them within direct attack range without cover (at the same time, flying units will be able to attack them diagonally from a safe distance). Large flying units will have speeds of 8-9 (enough to attack archers from any side). But they won't be able to reach the first row on the first turn (without tactics or skills/artifacts that increase speed).
Added 1 hour 2 minutes ago
Out of 70 dueling arenas on a 14x14 grid, only 9 are currently playable (2 of which are plain arenas without obstacles). I will remove the rest for now; they will need to be manually adjusted.