My "big" brooder is 4x8.....the "legs" are whatever you want them to be. I made mine I think 21" or so, tall enough to slide my feed tubs underneath for storage (which works GREAT).
The actual brooder area itself is 4x8. The back is full sheet of plywood. It is 2 feet deep. So each level is a full length, 2 foot "wide" piece of plywood. Each brooder is about 2x8......I used exactly 3 sheets of plywood to make the entire thing.
I had a thread with more info and pics from when we built it here
https://www.backyardchickens.com/forum/viewtopic.php?id=288978
I will say there are a few things I would change.......although having a board along the bottom edge helps keep shavings in....it needs to be TALLER and removable. Because it would be much easier to clean if I could just put a wheelbarrow under the opening and sweep everything out to clean it.
I also wish I had made the doors open either down or up instead of to the side. If you use it as a long single brooder per level instead of individual cages, it is hard to catch them.......they run from side to side and you have to duck between doors, etc etc.
I ended up installing those hanging plant brackets inside to hang a single 1 gallon waterer in there. Works GREAT. I could do the same for a feeder if need be, I guess.
I would love to have a small shed I could build brooders INTO the walls. That would be glorious. Someday I want nice 12x12 shed/stall/etc that I can make a permanent "brooder" room. I always have somebody needing quarantine or treatment or something. Right now I have a broody in one, a chick with an injured eye in another, and the shredded rooster in the third. Once I get the 4th scrubbed out (it is empty now but needs the walls depooped), I will move my BIG broody girl into there.
I will try to draw up some very simple dimensional drawings tonight for you. My ex was very belligerent about always being right, so we did a few things "his way" that were just stupid but easier to just do than argue about. Good riddance to him, LOL.