Most states treat a vehicle as oversize once it goes beyond about 8 ft 6 in wide, 13 ft 6 in tall, or around 53 ft for a trailer, and overweight beyond 80,000 lbs gross on the Interstate system. Those are common starting points, not a promise: every state sets its own limits, and some roads have lower ones.
The sure way to know is to check each state you will cross. If you are close to any limit, ask before you roll. A permit costs far less than a citation and a delayed load.
Not sure where your load lands? Try the load checker on our home page, or order a permit and we will check the rules for each state.