It's simple physics--the longer/larger the RV, the fewer sites it can fit in. However, it's not as though there's one "magical" length where suddenly you can't fit into lots more sites; a 32' RV is only incrementally harder to find a site for than a 28' RV.
My motorhome is pretty much exactly 32' long bumper to bumper. There are a fair few sites in some parks that I cannot fit in (particularly in some of the state campgrounds here in Vermont, many of which were built by the CCC and have small but private sites); however, I only need to find one site that can accommodate me, and thus far it has not really been a problem provided I'm willing to be a tiny bit flexible in planning. I would be a a fair bit more constrained here with a 40' motorhome. In many other areas, it's less of a difficulty.
The best suggestion I can give is to find out what the actual limits are at places you're hoping to go. In some cases the published length limits are rather arbitrary and have little bearing on the size of the site or the tightness of the access roads. In other cases, they're very much precise and a 31' motorhome just won't work in a 30' max site. In Texas and the surrounding states, I would not expect to find too many very tight sites; there's a lot of wide open, flat land, generally speaking.