As others have have posted there's no "one" length over 48' that will keep you "legal" everywhere. There are two primary considerations; when you're on a designated truck route (especially Interstates and major highways) you can run to the max. When you get off on secondary State roads you'll be subject to State laws rather than running under the 'Federal' umbrella. I've found enforcement of the laws to be choatic at best. It really seems to come down to "appropriateness"; that is, if you're 60 feet long in a State who's max is 55' but you're on larger highway, on a road leading to a campground, on a road leading to a major attraction and such the laws do not seem to be enforced (especially if you're an RV...money to be spent dontcha know). On the other hand, if you get a 70-foot rig stuck on some small country road in a State with a 55' limit things might not go as well.
I'm facing the same situation you are...my bus is 35' overall and I'm considering towing one of two trailers at any given time. The first one is a 16' enclosed cargo trailer (20' overall) that will haul the Jeep Wrangler and act as a workshop with the Jeep unloaded. That puts me at a comfortable 55' which you can go almost anywhere with. The other trailer is for a 22' C-Dory Cruiser power boat; it's about 30' in overall length putting me at 65'. This will work alright in many situations but if we travel in the Northeast I may have to pick my routes carefully. It's also not the rig I'd like to be towing when motoring through the Pennsylvania Dutch (Amish) country and trying to pull into a roadside stand or park at a quaint inn or restaurant.
There is no perfect setup; I want a bus with an accordian in the middle that shortens for travel and lengthens for living!
Out west here I'd drive the 65' rig almost anywhere as things are generally pretty open. I've driven a semi through Times Square in NYC; it's not a place I'd choose to take a 35' skoolie with a 30' boat trailer dragging behind; in fact, I don't think I really want to take the skoolie alone back there!
It really depends on whether you're 'traveling' or 'exploring' I think; you could run I-95 from Maine to Florida with a 65' rig and never have any trouble; especially if you stuck to stopping at the truck stops, restaurants and such just off the Interstate. But if you want to try out some of those small mountain roads in North Carolina, New Hampshire, etc or visit the Soaring Museum in Elmira, NY you'd be hard-pressed to deal with a rig that long.