I don't think there is any "standard" size out-there! Seems every campground we visit has something different. Some are huge, some are tiny. Some are made with metal pipe framing, some are so old they are falling apart. We had one a few years ago that was half burned. Yea, someone set it on fire and it was charged. Some campsites have 1 table, some have 2 tables. We've even had campsites with NO table. We have our own back-up tables we bring with us too. We even had a picnic table the required a person sitting on each side. If only one sat, the table would tip up.
I think you can plan on 4 people sitting at a picnic table. After that, anything is possible. (or should I say IMpossible!)