The idea of the open bottom tractor is to be able to move it around so the chickens can scratch at whatever you put the tractor over. With a wire floor they will be able to eat anything that manages to poke through the wire but they will not be able to scratch effectively and IMO might rough up their feet.
Right now my tractor coop/run combo (movable, but with no bottom), is stationary because I have to keep it in full shade due to the heat so I am using litter on top the dirt 'floor' as the forage is long gone with the tractor in one place, but then I let my chickens out for supervised foraging in a 30 by 50 fenced in area that is a chicken playground with lawn, weeds, piles of leaves, driftwood perches, shrubs, and a dust bath area.
If you want a wire floor I would go with very small diameter hardware cloth so they can walk on it without toes slipping through and would afford good predator protection (from digging predators) but small diameter hardware cloth won't let much plant life poke through. Kind of a catch 22. Why do you want a tractor as opposed to a stationary coop?
Do you plan to lock your chickens up each night in the coop? If so, you can move the coop to a new location before you let them loose and won't have a need to have them moving along on a wire floor, nor running them over with the structure.
Have you considered a stationary coop with multiple pastures so you can rotate them on the pastures? Do you have land size limitations?
One thing I suggest, if you build a tractor, have a roof overhang for shade and rain. When I had my run made I wish I had done that. Also, a hinged roof to be able to access the entire unit is good, with exterior access to the egg boxes.
