I would like to explain my chicken tractor - in hopes of helping anyone else out.
The doghouse part is approx 3'x3' and provides enough "sleep only" space for 4 hens. It was a large doghouse kit from Lowes. The cedar is britle and thin. I wouldn't buy it again, but it's cute and it does work.
There is also a nest box under the roosts, it is approx 1'x1'. I also added lawnmower wheels. I'm not happy with them at all. Not sure how to improve on that part.
I added sofit vents, and put hardware cloth over the gable vents:
The run part was built separately. I bought 3 - 8ft treated 2x4's for the ground contact part, and the other 2x4's were mostly scraps I had laying around. I got the welded wire free from my dad (also a scrap) and the tin free from my father in law, and I had the shadecloth top cover leftover from a dog project. Total cost for run: approx $20
I had to go and add chicken wire on top of the welded wire because the chicks could squeeze through the 2" holes. That added a bit to the cost, but no biggie really.
The demesions are about 3ft wide x 8ft long x 2ft high. Actually it's 27" high I think. I can sit inside the run with the chicks, but I'm an incredibly small person with a high tolerance for chick poop. There is approx 3ft covered by tin, which is where I put the feed and also provides approx 9 more sq ft of rain-proofed area.
The run doesn't have to be raccoon proof because I close the door to the doghouse at night. The run IS dog and daytime predator proof though, and we haven't had any trouble.
Here's another yard picture

I was worried about dogs digging in, because my backyard didn't have a fence. I used a small fence charger and a wire offset from the base with screw in insulators. I have a lot of extension cord, which provided power as I moved the tractor around the yard. The most curious neighbors asked me, not about the chickens, but why I moved the chicken pen every day or so. hehehe For them to have fresh grass, and so that it doesn't leave dead patches in my yard.
I designed the tractor to house 4 hens, but I ended up keeping a rooster so it raised 5 full size chickens to point of lay. We were able to fence the back yard by that time, so now they have a bigger permanent coop and free range on the whole back yard. We are currently using the tractor to house young chicks again.
To move the tractor: I attached a rope on the base of the front end. I would drag the run forward, approx 8ft and then push the doghouse on wheels up to the new location of the run. I always did this while the chickens were on the roost so they didn't escape.
Right now I just have another piece of tin to close it up, we aren't using the doghouse for the babies just yet. Overall, it's been a very handy design!
I hung the heat lamp from the rafter that I used to hang the adult's feeder from.

I realize there are not very good pictures of the finished inside - with roosts and all. Here is a couple of pix I just took with some of the youngsters sitting on the roosts:
I need to take the lamp out, they don't need it anymore. The Christmas lights are to help them get to bed at night, they go off 2 hours after "dusk" on a photocell timer.

The adult birds still sometimes lay eggs in their old nesting box, even though they have the new coop now, and they dustbath in the pine pellets. Here's a pic 2 of the Leghorns:

The interior parts are removable - they are based on a 4x4 and wedged into position rather than screwed. The big birds only managed to knock the rost down once, and I suspect a predator tried to break in. I removed them to convert the doghouse into a brooder!
Note the little crack where the ramp/door doesn't quite match up? Well one chick escaped there and died, so now I staple a piece of hardware cloth over the doorway when brooding.

Once the chicks got a little bigger, I moved them to the detached run and boxed in the tin side again to make an open air brooder. (The youngest chick in this picture is 7-10 days old, I like to put them on grass early!!!)