I run about 71 feet with a Ford F-150, so I worried about this when we first started, we knew we'd be traveling in CA because I had read on a thread here that California troopers "...have a tape measure..." So when we passed through the first inspection station entering CA in Spring 2013, I noted the long look I got from the CDOT person standing at the wave through point. Don't know what he was looking at, but he was looking in the direction of my truck and continued as I passed through, as I watched him in the rear view.
That said, spent two months in CA last year with no problems. Have seen a lot of rigs with trailers that where obviously pushing 80 feet. The fact they are out there and not getting stopped even though most states have laws set at 65' suggests no enforcement. I have not read of anyone having an issue if involved in an accident either. So I am a violater, and I guess I am taking a chance, but I have a lot of company
Might add, though, that there are certain roads in some states where they do have length limits, particularly in CA. I'd become familiar with those states, just to make sure you don't end up on a road you don't want to be on.