Of course, it's not just the length you'll need to worry about. You'll need one that can support the weight of those tractors.
You'll need to deduct the weight of the trailer from the overall capacity, then figure out how much the tractor and all implements will weigh and buy axles that have enough capacity for all of it.
Then you'll have to make sure that the tow vehicle can also take all the weight, especially being sure the tongue weight will be correct.
I agree that 20' would be a minimum, 24 would be almost ideal, but then that changes a lot. Rules on overall length can make you have to have a different driver license.
Many variables come into play.
John