what is the most common cargo trailer in use?

Trailers are according to freight in the USA. There is no one common or most common.

Europeans have a heavier axle rating but limited length issues leading to cabovers there and here we are up to 65 feet on some roads and 70-75 on others depending on what you are hauling and what vehicle. There are all sorts.

It also changes with what I call eras. When I started 40' containers were max. Then 45 then 53 and then subdivide for domestic and international. Box trailers go out to 53 generally, I started again with 17's and worked up to 53.

There are sometimes combination vehicles that go into rocky mountain and turnpike doubles and a few triples. NY Thruway has the turnpike doubles which I think are two 48's but probably two 53's now. And have the horses to do it.

Again there is no most common anything in trucking. The industry itself is common but not unified as in divided needs depending on the freight itself. And sometimes the seasons would require use of not so common trailers like reefers to heat brass coils for small arms at Remington. To keep them stable and use on arrival. Otherwise box trailers for them.

Certain trucks are evolving beyond common. You got a 80000 pound tractor trailer on 18 wheels and 5 axles. Great. But you will see a Mack Long Pump Cement high tower at 110,000 as it sits on 6 axles plus one tag when everything is folded up between jobs. Other trucks go beyond. Into the rarified world of heavy haul, it would be possible to transport say 600,000 pounds and some equiptment for mining etc do not do trucking at 2 million pounds but walk on the earth itself with support from a fleet of rock trucks to protect the existing roads it must cross. Most of our interstate highways will tolerate military traffic far exceeding 40 ton such as M1a3 tanks at 75 ton and beyond. They are their own system and not subject to the usual permits, curfews and so on. One of our overpasses over a US highway (Future interstate in a few years once completed to that code) carries at least a 150 ton transformer weekly at times on many axles.

 

most common size would be a 53 foot trailer if that's what you meant