DM + 6-8 is ideal for me, when I run. We have run with larger groups so long now, that is where our skill set is. In particular, we have gotten really good at having the players roleplay with each other and other such tricks to keep interest up with a large group. When missing people, some of that goes away. I can run 6-8 with no sweat. 5 or 9 take my A-game, but are doable. Outside that, it's not as much fun, though I can handle 4 or 10-12 in a pinch. 3- or 13+, forget about it.
That's at the table. My group has stayed at 9+ players for some time now. But with that many, someone can't make it almost every session, and we will get the 6-8 almost all the time. Of course, when we had 11 players, I only got stuck once running for 10.
With a group I don't know well, I prefer the 5-6 range. The players typically don't have the skills to play well in a large group, but I've mostly lost the skills to make 3-4 interesting. Not that I'm that good with pickup groups, anyway.
When I play, it doesn't matter. I can have a pretty good time even with a DM in over his head, and half the players as idiots. So if it happens to be optimum numbers, so much the better.
