As the the title states, is there a maximum size for generated structures beyond which they don't generate properly?
You can just make a sectional and use the boolean tiles in tiled to repeat it until it hits empty background. I remember doing something like that on ocean worlds back in beta
I believe if you create a structure as a dungeon or microdungeon you can set the maximum generation size and maximum parts number. Beyond even that, from the size of the png that mapgen creates, it looks like if you want to hand make the entire structure, you can make a structure the size of the entire planet. I may be wrong though.
If you link a bunch of microdungeons it changes the world generation depending on the angle you approach the anchor point from. At least it did that when I tried to split a structure into to parts for someone back in beta. If you walked from the left everything completed but walking to the right left half the dungeon out!