Our stalls are built from a lean to off the side of a shed. They're 6' wide x 6 ½' deep x 6½' tall at the lowest point. These guys right now are about 35" and 36" tall. They shouldn't grow much more. Here's the setup for now:
They're roomy enough, but still cozy enough for them to lay down to sleep at night, even though they're pretty open right now. They seem pretty happy with them though, they even have enough room to roll when they want to.
Our horses obviously don't mind the cozy sized stalls, but it probably depends on the horse how much room he "needs." If yours is used to a bigger run in or being in a regular size stall, normally 12' x 12', he might want more room. We made our doors, but you can buy metal, wood, plastic or a netting style gate or door for your stall. There's a lot of choices and sizes. Ours are as wide as the stalls, so 6' each, and 3' tall.
You didn't say how long your shed is, but if you're going to build a lean to type stall off of it you might as well use the entire length. You could have a large stall for foaling, 2 stalls if you decide to get your horse a buddy later on, or a supply room next to the stall if he's got plenty of room and is happy in a smaller stall.
Before it gets too cold, our stalls are getting a redesign, though.
We're going to add stall mats, dutch style doors, and windows we can open and close as necessary depending on the weather. (We're in Indiana).
We want them to always be able to see outside, and get good airflow when the weather is mild, but they'll need more protection from the nasty weather we get every winter, so we'll be getting started on that soon.