There is a Federal Law that sets the standard. look in the US Code it is covered in detail there.

BUT the way our Constitution is written each State has the Right to Add to those laws as they see fit, but not take away from. therefore writing their own laws in conjunction with the Fed Law.

Last time I read the law on size for vehicle 53' was the Max period. That was in the Federal Law (US Code). Anything beyond that requires a special permit.

Just like all of us weekenders running around with triple axle enclosed trailers that gross over 10k lbs. Your Illegal by US Code without the right license. Class-A equivilent for your state.

Not many states actually write their own Laws when it comes to this topic. Most just use what is in the US Code and their own interpretation of it. Some even go as far as a guide on how to interpret these Fed Laws for their Enforcers.

So if you are worried you might get nailed or want the laws changed or clarified try talking to your Senator/Congress-person. It won't be the first time there has been a push for it.