Size of tractor is less important as moving it regularly. Keep them on fresh grass. MOve every 1-2 days so you need a design that you can move easily.
I made a wooden one and it is very heavy and not easy to move, so it doesn't move. I let that group out to free range instead. The land here is very rocky to boot,a nd is not condusive to my style of tractor. THough someone suggested a PVC type is likely to adjust to the contures better.
Keeping the tractor smaller will also help with the design. THe larger it is the more supports it will need = greater weight.
Invest in the legumes-- alfalfa and red clovers grow tall; white clovers tend to be lower. I did some reseach last winter and found that finding this information is difficult.
I read all the Joel Saladins books I could get via library loan; I read the blog at sugar mountain farm in VT. The latter raises pigs but if you read carefully youwill find what he seeds his pastures with. Realize he is raising pigs so a chicken cannot always use what a pig uses -- but it got me to think out side the box.
apple trees, choke cherrries, grapes, beets, peas, many types of grasses, amaranthe.
My chickens loved eating the yellow squash but left the plant itself alone. THey stole grape tomatos off the tomato plants.
Every state has a county extension agent-- look in the phone book-- make a phone call. Ask what varieties of legumes are good for yur growing area. THink of a way to have grasses all year round if possible.