Maximum size is something you just have to get a feel for from experience.
I try to choose sites which can take the full length of the trailer and TV - 58 feet. But since over half of our camping is in COE, USFS, NP and state parks - that isn't always possible. Often I can put the TV partially alongside the TT.
If I'm going to a new CG and am concerned about size from the website/ reviews, etc - I try to look at the CG on Google Earth/ Bing Maps. It is looks really tight and twisty, I'll go someplace nearby - and drive over in the TV to be sure. I've even paid a gas station $5 to let me park the trailer there a few hours while I looked over a campground. Since I bought fuel when I came back to get the TT - he didn't want the money.
Pictures of individual campsites are really useful. I note that the CG in Yosemite Valley almost all have a big rock at the back of the campsite so you can't go past the pad with the overhang.
Also be careful because sometimes size restrictions are because of the roadway leading into the campground, not the actual campsites.