There is an interesting level of challenge to be had in playing in the cramped environments of the smaller maps. GalCiv2's AI deal with it much better than most. That said, my current game is on a large map, and I may go to huge for my next. I mix up the settings from game to game (really wish there were random options on the game creation screen). I avoid abundant planets, which results in a galaxy full of stars with exactly 5 planets around them, which is annoyingly silly. I stick mostly to normal tech, I pick scattered clusters a lot, and tend toward abundant habitability. I usually play with 1 more AI opponent than the default for the map size, though going with fewer lets them spread out more and can lead to an interesting clash of the titans scenario.