I think it may be difficult for anyone to let your know which is the largest; there are over 120 preferred sites and the entire site, including the tent pad, may not even be visible from the road if it is occupied. Do you plan on erecting your tent on the paved pad or are you planning to utilize the sandy tent pad? (Puzzled about your boy scout comment???)
Even if you do request a specific site, there is no guarantee whatsoever that your will get that site. We've never requested a specific site number, but do request loops, which is usually granted, but not always. CMs do their very best to make magic, but it's not always possible. Did you specify your tent size when reserving?
We do have a motorhome with three slides and an awning. When we are fully extended, our driver-side slides frequently extend out beyond the pad. FWIW, we have found that the preferred sites are some of the tightest we've encountered at the Fort, and the roomiest, the full-hook-ups in the 1600, 1700 (favorite), 1800 and 1900 loops. We've never had a bad site!
I suggest you be sure to note your tent size on your reservation. When you check in, ask for the largest site available at that time. Inventory changes all day. CMs have large notebooks with the site notes, including sizes; we've been given our choice of open sites many times (we also like to store our golf cart trailer on-site when possible, so like a long site) In September, you should have some choices (even though the preferreds are more limited in number). Even then, if you drive to your site and it is not to your liking, go back and request another. CMs want you to be happy and will bend over backwards if you are pleasant.
And if you really want to drive yourself crazy to find the perfect site, there is an app for that, complete with videos.