This is one of those issues that you will never be able to satisfy everyone and everyone has a passionate opinion. We have had a similar discussion on our board http://www.deschutescounty4wheelers....?TOPIC_ID=1764
. There are some updated USFS specs posted there. In the Deschutes National Forest, the forest supervisor has adopted a blanket 80 inch rule for all new trail systems. Keep in mind that a full-size F-350 is under 80 inches. Depending on the vegetation, terrain and distance between trees, sometimes a length restriction would be more appropriate. Sometime a tire size restriction is more appropriate. Sometimes a width restriction works better.
The goal is to maintain the integrity of the trail system and not allow folks to make a freeway out of it. Often times, people lose sight of this goal while "protecting" their right to use the system. Not all trails are appropriate for all rigs, we just have to accept that. A full size truck with 44 inch tires is inappropriate for Edison, whereas a stock or semi-stock rig with 31" tires and no lockers is inappropriate for Santiam. We need to keep the big picture in mind.