There are a couple/few campgrounds in Yosemite that do not allow popup or hybrids unless you can "turtle" it - meaning you can camp in the RV without opening the softsided parts. That usually leaves tent trailers out, but will let hybrids and volkswagon camper vans in. In general, unless it's posted (or stated when you go to make reservations), you can camp with a softsided RV. In general,if tents and RVs are allowed, there is no restriction to hard-sided only.
As for RV sizes, as others have mentioned,smaller will get you into more places. From what I've seen, there seems to be some standard breaks: 18 and under; 24 or 25 and under; 27 and under; 30 and under; and all sizes. This isn't hard fact, just my experience.