Hi Debi!
Oh, I love Fort Wilderness! Everything about it is just so...secluded! It is like being in another world--even another world outside of Disney World! It's one of my favorite spots.
That being said, I've never actually stayed in an RV. I've done the cabins and an actual *gulp* TENT! (I was 3 years old, so I don't remember much about it...)
I did a little research and the Disney website doesn't tell much about the size of the campsites, just the stuff that is included (electricity specs, etc.). You can
see that page here.
I also went to allears.net (my fave unofficial spot for mega Disney details!) and searched "fort wilderness" which brought up a great page that included details on campsite sizes (about 25 feet wide) and info on the Premium Sites (like they accommodate vehicles 45 feet long with multiple slide-outs). So check out that page and I think you'll find what you're looking for!
Have a magical vacation at Fort Wilderness!