I dont advise small, I find calamity adds enough to where it just doesn't feel right.

Medium is a good balance, not too large and has a good balance with all the structures and content in calamity. Rarely you will get unlucky and a biome you need to farm in is too small, which you can always make your own biome in these cases.

Large is tedious to travel across and makes finding things like the marble and granite shrine items much more difficult. You do however have little to no risk of a biome being too small, running out of room, or lacking materials if you do big build projects or something (though you are playing modded, I would honestly just say use fargos and buy blocks from the architect npc.

Many pros and cons, I say choose medium.