It is not too hard if you live somewhere with beautiful weather 365 days a year and are going to free range the chickens (so the tractor is merely for nighttime housing, they will have free run of a large area all day every day). In that case all you need is something big enough to contain 6-8 linear feet of roost.
It is also not too hard if you live somewhere that does not get freezy winters or lots of bad weather, and either the tractor will be located somewhere you are willing to bet is totally immune from major digging predators like dogs or foxes (e.g. if it'll be in your totally-privacy-fenced backyard and you don't own a dog yourself) or if you are willing to play predator roulette and cope with maybe losing much or all your flock some night. If that is your situation, you can do an 8x12-ish cattle panel or pvc-based hoop coop with one end enclosed (tarps or tin) for shade/shelter, with flip-down aprons to prevent minor digging in.
If you live somewhere with lots of nasty weather (cold winters, or lots of wind) and/or are not comfortable with a not-especially-predatorproof setup, then your only options are the following: 1) crowd them like crazy and hope to have survivors for a reasonable length of time; 2) build something that requires a lawn tractor to pull it; 3) build a two-part setup, where the coop and run are separate units that you move separately (possibly requiring a lawn tractor) and dock together when they get where they're going; or 4) have fewer chickens or don't use a tractor.
(oh.... unless these are meat chickens you're talking about growing, like CornishX that will only be there for 6-8 wks and then to freezer camp; you certainly CAN build tractors adequate for that, go check the meat birds etc section of this forum for lots of examples)
Good luck, have fun,
Pat