We were at a Thousand Trails CG this past weekend in WA. Was really lucky to be able to snag one of the nicer sites (for the 2 of us). Man, it was HUGE! Enough for 2 RVs and still have enough space at the rear for a few tents. Their policy is max 6 persons per site but have never seen them do a head count and have seen some pretty large families. Not sure if they charge extra but I wouldn't tell them. If you had a party of 20 and were causing a disturbance they'd be all over you tho.
Some of the T/T CGs have teeny weeny sites as well. If you want one of their nicer sites in the high season, it's going to be pretty tough, esp. if you want one with sewer. Their sites are all first come first serve. Being members, we don't pay anything per night for a site.
You might try doing some googling for area CGs and/or look at CG review sites like RVparkreviews.com. To find out if they have any larger sites, you may have to do some phoning. Online CG maps aren't going to show what individual sites look like. Maybe google earth? The occasional CG we've been to has a family/group area where you can get adjacent sites.
I don't buy that more people on a site is more work and therefore should pay more. Have yet to see trash left on a site where workers have to come and clean it up but we don't stay in "those" sorts of CGs... :)