I found the system editor bit, but not the server script... I want to play on a size 2000. (Don't you worry guys! I'm using another computer at the moment, 16GB of RAM, Quad Core i7 (Octo with hyper threading) and a NVIDIA Geforce GTX 860m ) Anyway, I'd love to know which javascript file it is, I tried searching, but couldn't find it.
sim_utils.js, circa line 30. If there is no additional engine limitation, it looks like you can also override the allowed range in the biome type definition.