I prefer very large maps usually since you have time to get to some of the interesting lategame tech while there's still opponents to use it on, but they do take forever to beat on transcend just because moving all those units extends each turn massively. When I don't have a few days free, mid size maps are ok. You do feel more threatened early which is fun, because the AI's cheating is harder to counteract when there's only a few units in a war so you can't mass kill stacked units easily.