At Grant (not stayed in Madison) the few times I have been there not had a problem. You don't know what type of site you will get until you get there it may be pull through or back in. From what I recall 6 inches is not going to make a difference. The last two times I was at Grant I ended up with a pull through that could have accommodated a longer trailer with no problem. However I had to go in backwards due to the tree placement in the site for my slides to extend (which they dont care about as long as you have someone working the traffic as there is no space for passing around an oncoming vehicle, obviously the back in sites that will make no difference.
If I am unfamiliar with a campground and can't get good info on it and they have the availability I have no problem reserving the next size up.
In any case I don't think you will have an issue from what I can recall about the site sizes. They are not overly large at all but again I don't recall seeing anyone having a problem at least once they got in their site. Also I don't believe they are counting on your rear overhang going over the rear backstop as part of the measurement however there may be a tree right there as well.