Six lands is a very nice map for six players
Desert Eye is a map I personally like a lot, although it might be a bit big for six players. You could of course add in 6 AI nations to make it (very) crowded and give each player an AI opponent, or take it easy and start spaced out with just the six of you (at least everybody gets should get some proper research done and get into the world of summons and items in earnest
).