By Subaru Outback with the 16' Scamp
was 32' and I think the truck and trailer are at about 34'.
From the Park website & FAQ section:
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.