Dunno if there is a max. size. I guess as long as it can fits on tables it's good. I guess it also depends on amount of players, amount of units,...
Beginning terrain building, I haven't built much yet. But i know i prefer things that fit in almost all maps, like trees, hills, fences, buildings that can function on their own. And mostly parts that can be moved, so that it creates lots of options for gaming. I would like to have a gate with a high wall and stairs tough... But i don't think i got the good equipment for it. Bit low on money to buy me stuff to. But i got lots of cardboard and isimo.. Maybe u could write something for me about using that. It's taking in Two of my shelves at most. They should be filled up with terrain!