I have a 26' Class C and 31' CE (building). We have traveled about 40,000 miles in the Class C and have found that for the type of campgrounds we like, 26' is where restrictions start. Glacier NP has a restriction in place. Most individual campgrounds that do have restrictions, have alternatives near by. I have seen very few private campgrounds have restrictions, but we tend to stay away from private campgrounds - we only go there when in between or it's the destination for some reason.
For the most part, you will not find that restrictions impact your journeys.