A question for the map makers: How do you make the 15x15 size?

With vanilla map editor you can't - the game enforces arbitrary limit of 64km^2 as a hardcoded max area, even though the actual game engine supports maps 4 times larger than vanilla (16x16) by area, but it needs a mod to remove this custom limitation. The 16x16 limit is much harder to overcome due to how code is done under the hood.

I guess they put it for performance reasons for weaker hardware.

I have managed to vibe code such mod and i am creating 16x16 map. Can share if you like.