What is the largest tires that people have used with the longfields?

My brother uses 42's with them.

38.5's here..

Bobby doesn't specify a limit to tire size. I am on 38's and they have held up as expected. I don't "mash the gas" to clear obstacles, but sometimes you gotta "get it"!

38.5x14.5 here also. It is nice not to have to worry about blowing a birfield when gettin' it:)

Is a longfield the same thing as those recent "super" birfields?

yes they are the same thing.