When in doubt, find the campground in GoogleEarth, and use the Measure tool. Depending on tree cover, most of the time you can find some of the parking spots, so you can measure them. I never rely solely on campground descriptions, as terms vary in meaning.
Glacier NP has the length restriction from Rising Sun campground up to Logan Pass. This is on the east side. There likely is the same restriction from some point on the west side, but there is more road from the west entrance to where the restriction would likely begin. We camped at Rising Sun campground, and there is room for some longer trailers in various sites, but not all sites. One loop has larger sites than the other. No reservations at Rising Sun, so you must be there before 10 a.m., normally, to get a site.
The St. Mary campground is just inside the east entrance, and has more long sites. They do take reservations, I believe, but we chose to stay at Rising Sun. Drove through St. Mary campground, and it was okay, but Rising Sun fit our tastes better.
Enjoy your trip!