Thanks everybody. I have secured somewhat odd old metal trailer (it's made out of metal pipe) to build my chicken tractor on. It is 3' 6" wide X 7' 10" long, which is just about perfect. I am planning on securing 4 x 4 inch beams to the pipe as a base to build onto, which will also enable me to widen and lengthen it a bit, I'm figuring 4' X 8' will make it easier to use 8' studs to frame it out with. That will also give me 32 square feet, enough to house a couple more chickens if I want!
A lot of the chicken tractor designs I see are using lawn mower wheels, which I think would sink into any soft ground and be awfully cumbersome. This makes much more sense. I will post some pictures as I go along.