It really depends where you are and what sites are available. In many cases, it is not a significant detriment. In some others, it may very well be a significant constraint.
That being said, if you are looking for sites with hookups, you will generally have no trouble at all finding large enough ones; it mainly is a problem in older, often public, campgrounds which were laid out before large RVs were common. Many of the Vermont state parks are examples; in quite a few only a fairly small percentage of the sites will accommodate a 35' RV, and if those few sites happen to all be reserved or taken, you're out of luck.
My motorhome is 32' bumper to bumper and, while it has been a limitation a few times, I have never been unable to find a suitable site when I wanted one, though I have had to be a little bit flexible a few times about where I wpuld go. On the whole I wouldn't worry overly much.
It's also true that, no matter how big or small your RV, there will be some sites you just cannot use; and naturally the larger the RV, the more sites you won't fit in. There isn't really one length where it suddenly stops fitting, but a gradual progression. (Probably the biggest single step down in availability occurs when you go from a tent, and hence can use any walk-in site, to using an RV of any sort that must have drive-in access.)
