I'm not sure what kind of campgrounds they are that you're looking at but if you look for national forest service campgrounds, here is an excellent source and they will give you the site size range:
Campground Directory and National Forest Campgrounds
Many times you can Google the campground and get additional information or pictures - even for city or county campgrounds.
If you have an address, get a satellite view of it.
RVParkReviews.com is another source for a general idea. Look to see what kind of RV the reviewer is using.
There are also some sites that will give you a photo of each site but not all campgrounds are included. Here is one and you can Google for 'campsite photos' for more.
With our 40' motorhome we used public parks 90% of the time and it worked for us. You just have to look at various sources.