As for party size I'd echo the 5-6, 4 works really well too. But you want to keep it smaller just so that you don't have to sit around waiting for one player to post. Granted you'll run that risk no matter what size you have, but the smaller number the less chance. Plus I've noticed with smaller partied campaigns that the group becomes more cohesive as a team, both in roleplaying and everything else. Whereas with larger groups you can get almost cliques in a sense, where some characters interact more often with others and not often with the rest. It's odd.