Building a horse barn from scratch is the perfect opportunity to customize every option you want (well, actually, that you can afford).
First of all - Build your barn as if you expected to keep wild elephants in there. A horse is a VERY powerful animal and can knock or kick down wall easily. Same goes for the hinges on your stall doors. A thousand (or more) pound horse leaning on them for hours everyday takes it's toll.
One of my favorite horse barns was a smaller one I built from the ground up and I just loved it. I even made it match the house!
It had a simple layout - two stalls, a feed room and a tack room. The whole thing was 24' wide by 48' long. I oriented it on an east/west axis with the stalls on the north side. Even though I live in a warm climate, I still like to be able to close the stall windows to keep the cold wind from blowing through in the winter. This allowed me to keep the entire south side open and only have a half-wall along the alleyway. Horses need good air circulation to stay healthy.
Click here for some pictures of this barn.....
I've always preferred a stall size of at least 12'x12'. If you have smaller horses, you could get away with 10'x10', but remember every horse needs a nice place to stretch out and snore. I found the 12' size to be optimal for several reasons. It gave plenty of room even for my 17 hand QH/TB cross. And it was easier to clean and keep well bedded with shavings than a larger stall. Multiples of 12' also work well with building materials which is a big consideration with wood getting so expensive. A 12' wide alley way gives you plenty room to tie your horse up and groom whether you crosstie or single line tie.
I also love stall mats. They make cleaning a stall so much easier. Your horse never gets to dig holes in their stalls either. Unfortunately, they are expensive. I have been lucky enough to scour around and find rolls of thick rubber conveyor belting. The worst part of this stuff is it's HARD TO CUT! (oh sorry, did I just shout?)... and it's never in the exact size you need width-ways to make an exact fit. But I guess at the price I paid, I could put in some elbow grease. I'd also put a single long piece down the alleyway of the barn so when a tied horse got impatient and pawed, he wouldn't wear out his hooves on the concrete. It was a lot easier on the concrete too!
If you have the room, consider building a run off the back of your stalls. They are nice features to add and your horse will have the option of walking around a bit and getting to have a different view of the world besides just looking out his windows. This is especially nice if you have a small property and can't let your horses out in the pasture for long periods of time. I've used several versions of runs in the past including building a stall with a large attached shade overhang. That shaded area opened into a large run that also had a gate at the end opening into the pasture. This was really a great set-up and gave me a lot of easy options for turnout.
If you're not sure what type of barn you want to build, drive around to other horse properties in your area and look at what they have. Ask the owners questions about what they like and what changes they would make if they could. Also look at the layout of the barn and pasture fences. Good planning can make the most of even a small property.