How about not picking the map, but have a map generator that creates a map from scratch, following the specifications (like size, terrain resource deposits) you fill in. I think Cossacks (not C2) and American Conquest had this too, and it worked great. Apart from that, those games also had nice map sizes, some of them being enourmous. Another good thing was that I never reached the pop limit, I think the hard unit limit was set to be 64000, so whith 2 player each player would have a limit of 32000 units, with 4 players each player would have a limit of 16000 units. Still, no limit is even better.

EDIT I'm not sure if those games had a collision detection, I think not, but it never bothered me. And these games needed a high pop cap, as a mine could have up to 95 peasants inside, while a formation could be made of up to 512 soldiers/marines/etc.