I loved the 'tractor' coop ideas too when we first started, but I quickly considered that the 'flat sides' had to meet 'flat ground' and my ground was gently rolling and even in the flat zones, was uneven (not a residential lawn) and the young birds might escape, predators could get in during the day and all manner of oops's.
We had an older coop that we used for a few years, then finally built one with mostly recycled materials.
We built a 9 x 12 coop with a yard about 5 times that size and it served us for many years.
Putting a wire roof on the yard was probably the biggest job, but the most satisfying when I would sit by my birds and see the red tail hawks sitting high up, drooling but unable to get to my birds.
Ya me!!!
The wire walls didn't stop a bear one time thou...but not much will.
And we thought it was a young bear because we never had that sort of trouble again. Whew.
Our municipality had rules as to the size of outbuilding, and how big before it required a building permit....erg....
a 10 x 10 shed is commonly for sale at any hardware type store, but here, the magic number was 108 sq ft.
hence the 9x12.
A light or two for winter, high and low perches for the birds, good ventilation and a consideration for clean out (a large flip up door flush with the floor made that job easy peasy) were our best options.
If opting for a big and small door, just do one size....sigh...silly me. (for the birds that is)
And if keeping in the winter, if you have enough birds, they will be comfy enough without needing a heat source (I'm in Eastern Ontario, and it's rather cold in winter here...lol)
If you have too few birds, they may get cold, but a low light bulb can help there too.
Good luck, have fun, read lots, and practice anything that makes sense to you til you find things that work for you, your birds and your style of keeping.
Not everyone's advice works in every situation.
Cheers
Jenny
oh...and consider a dust bath box or something, for the hens to keep themselves and feather's healthy. Read up on mite control. That can spread fast and can also be fairly easily controlled on a maintenance basis. (we used some sand and ashes mixed for them)
New birds, keep separate, inspect for mites, dust if you think you want to be sure, So, in building, consider a quarantine area, for newbies, or for sick ones to keep separate from the rest. Sounds a bit of extra work? It's worth it when you need it. And works great when you use it.