On the National Network, states must allow vehicle widths up to 102 inches. They must also allow a semitrailer of at least 48 feet in a tractor-semitrailer combination and twin trailers of at least 28 feet each. There is no federal height limit, so legal height is set by each state.
This is why 8 feet 6 inches is the legal width almost everywhere, while legal height and some length rules differ between states.
Regulation: 23 CFR 658.