And don't get too hung up about specific sites and sizes.
The Fort stays "very full" except for the summer season (June-Sept) when it's only "kinda full" so most times you don't have a lot of choice when checking in and getting a site.
I like Preferred sites (like you) and there are only 3 loops (about 100 sites). During busy times, there are maybe a dozen+ sites coming open (100 sites / 7 days a week). Depending on what day you arrive, some of those (half) have already been claimed (checked in and set up on). Now we're down to seven. Some folks plan moves and take some of the remainders out of inventory (been hit by that). Some will be claimed by folks who use the Early Online Checkin option in
MDE (they usually list an early checkin time) and get pot luck assignments but that takes half of the remainder maybe.
So you go from seven minus move folks (people in Preferred sites already who want to move to a "better" site) so subtract two and then subtract two for the Early folks. That leaves three sites available (hypothetically) in the Preferred category for you to choose from when you arrive.
You can only take what it available. That's why I say, out of 100 sites if your choice is among 3 sites, don't get too hung up on specific sites, measurements, locations, etc. If this is a concern for you, list the size of your trailer on your reservation as a note and the Fort CM assigning/reviewing will generally make sure your rig will fit on the site assigned.
It is what it is. Good luck!
Bama Ed