The Fulls are 1600-1900 and 2100 as indicated above.
Campsites are assigned a few days before arrival based on expected vacancies and departures. Fort Cast Members are very cagy and wont tell you what site you are assigned to until you start the check in process. An earlier guest may get swapped to your pre-assigned site before you arrive and you would never know.
The question about specific sites comes up often. You can ask if a specific site is available when you arrive and even request it in your reservation but if it's not available you won't get it (after all it's just a request). If you requested a specific site and it wasn't available, the Fort has no idea WHY you wanted it and thus can't give you a similar one since they don't know. Some sites have "ratings" such as "easy to back into" or "close to Comfort Station" but whose to say what's easy or close? One person's easy is another person's hard (we've heard that type of gripe here too which is why I bring it up).
Some of us have favorite loops or positions such as "the outside of 1600" or "near the 1500 bus stop" and the CM will do their best to match you up. But you don't have time to run through a list of 20 campsites checking if one is available.
In the Full category, some loops have general reputations. 1600 is the closest, 1800 has some pretty waterfront sites on the canal, 1900 is quiet, so forth and so on. I suggest you get some info for a loop recommendation based on what your preferences are, call and add that request (and it's not a guarantee) to your reservation, and take what they give you if it's in the loop you want. During your stay, check out other loops and note a handful of sites you might want.
For example, I like Preferred loops up close to the lake and the boat to the MK specifically loops 100 and 200 (300 is a pet loop and we don't have a pet). I've stayed in 100 three times, 200 once, and got placed in 400 (a Premium) once even though I had requested a Preferred. So I am pretty familiar with those two loops and have an idea where most sites are or what they are. So when I check in I usually have loop 100 specified in my request and when I get assigned a site I usually ask for one or two specific sites in 100 if they are available. If they are great; if not, I stop at two and take what I got.
The odds of requesting a specific single site are pretty low but it can happen if you check in early in the day in a non-busy period of the year possibly mid-week and are willing to wait if someone has checked out but the site has not been prepped yet for the next guest. Any of those things can help your chances.
But it can happen. So I suggest you do some research and add a loop request to your reservation and shoot for that.
Bama Ed