This is a frequent topic & yes, size does matter. The bigger you are the fewer the sites that will be available. It is as simple as that.
It is tough enough in CGs with only a few sites long enough for a big rig but the problem is made worse where a CG lets you pick your own site & you drive around the loops only to find TCs, class Bs, small TTs & class Cs occupying the few sites suitable for large rigs while smaller sites sit empty.
That is the way it is. Bryce Canyon NP is the only one where I have seen size designated sites. Even there I drove through looking for a site suitable for our 34' 5er only to find a class B in a site clearly marked "Over 30'". Another time found a Volkswagon Westfalia parked in a 60' backin site in Teklaneka CG, Denali NP, AK.
We did our RV search knowing that our trips would be multi month & knowing the size issues with SPs & NPs. We had set our size limit at 32' because of this but eventually bought the 34. Glad we made the effort to stay in the mid sized 5er range. More than 1800 nights on the road & hundreds of SPs & NP nights later there have been few size issues, even in CGs where they say 30' max.
We have however, seen many instances where only a couple feet longer would have made the difference between fit & no fit in site. In many cases the DW spotter has made the difference between getting into a tight site or not at all.
Boat: 32' 1996 Albin 32+2, single Cummins 315hp
40+ night per year overnighter
2007 Alpenlite 34RLR
2006 Chevy 3500 LT, CC,LB 6.6L Diesel
Ham Radio: VP9KL, IRLP node 7995