Average size? All I can say is most of them are plenty big for even a big tent…and most are very nice compared to the BWCA campsites. Most have pea-gravel or sand and are very comfortable. Also, latrines, bear-proof container, and picnic table is generally standard unless some idiot decides to destroy them for whatever reason.
We stayed at a resort on Kab but we did go out and check the sites out. They are very very nice and I will be doing that trip when the kids are big enough. It appeared that our 8 person tent would have fit on the few tent areas we saw.