Laws differ by state in some ways. For example, in some states 8' 6" width is allowed only on some designated roads, otherwise it is 8’ 0".
And, aside from what is legal to operate on a particular road, do states have discretion regarding what they allow in a vehicle that is registered in their state?
Rederal laws don't generally give a set hard and fast size that states MUST use. They give minimums and maximums, States are allowed to legislate that state minimums are greater than federal minimums or state maximums are lower than federal maximums.
So the issue is when one state has a maximum at fed max, and the next door state has state maximums lower than fed maximums.
So if Fed says 9', State A says 8'6" and State B says 8'. Then 8'6" State A Trailers can be used on the Federal interstate in all states but not on State roads in State B.