scanmom,
I got back this week from a stay in a Preferred site. There are a very few oddball sites (don't get #201) but nearly all are at least 12-14 feet wide (the width of the concrete driveway is 12' and the tent pads generally are 1 ft wider on each side or so). Depth is the variable but I think a 10-12 foot deep tent would fit. There is an issue with the charcoal grill being in the tent pad and would be an obstacle to setting up.
So generally speaking a 12x12 or smaller tent would fit most. You can ask when checking in if the site Disney assigns to you will fit your tent if it is bigger. The Cast Members checking you in at the Fort have access to a big 3-ring binder book that lists the dimensions of each site (and their tent pad). So if they have to they can look to see if it will fit.
Bama Ed
PS - I saw several RVs in the Preferred loops that put their back tires at the back edge of the pavement and had a couple feet of RV hanging out over the tent pad. So how big your RV is and how long the paved driveway is for it weighs into this too. It's just not all about the size of your tent.