Hi Phil
I have an old sideboard in the hen house that I use for my broody hens. Someone was throwing it out and it is ideal as a brooder and or infirmary. Sometimes I have a broody nest in each end and a communal food/exercise area in the double doors middle bit. It stands on legs about 10 inches long and the body is about 18-20inches high and about the same deep and it's about 6 ft long. If you have just one broody, an old bedside cabinet on it's side with the drawers removed should provide her with a secure place of her own to incubate eggs and spend a few days with chicks before they are ready to go out into the big wide world. It could be chocked up on bricks and the door used as a little ramp and you can make a simple mesh screen to replace the door. You could probably even place one of the drawers back inside to make her nest. I find old drawers make excellent nest boxes for layers as well as broodies.
If you are talking about making a completely separate coop for a broody....think rabbit hutch dimensions.
Hope that gives you a few ideas.
Regards
Barbara