Copied from the Glacier National Park website:
Are there vehicle size restrictions on the Going-to-the-Sun Road?
Yes. To help reduce congestion, vehicle size restrictions are in effect. Vehicles, and vehicle combinations, longer than 21 feet(including bumpers) or wider than 8 feet (including mirrors), are prohibited between Avalanche Campground and the Rising Sun picnic area parking. Vehicle and vehicle combinations over 10 feet in height may have difficulty driving west from Logan Pass to the Loop, due to rock overhangs. Stock trucks and trailers are able to access Packers Roost on the west, and Siyeh Bend on the east.
There are no campgrounds beyond the St. Mary area on the east side of the Going to the Sun road. Avalanche Campground is the furthest in on the west side.