It'll always be a set size, but that size will be configurable by the player. The map can be so large that it's effectively unlimited, here's a stress test of a 2,000,000 tile map https://www.youtube.com/watch?v=xsgn9Re6J_I
Holy cow. I assume that was made on some super rig and my normal pc would either grind to a halt or melt into a glowing green puddle, right? That is pretty impressive that it would run on anything at that size.
The bottleneck only really comes from drawing lots of stuff on screen at once, like thousands of dwarves. Just having the gigantic map in memory is fine, though of course it takes a bit longer to generate than a normal one. This might have been nearly a minute.