I just convinced my own parents to build me a stable, but I'm paying part of it and my plans aren't so extensive.
-If you're on a limited budget, start off small: say, put in a pole building of the size you desire and an outdoor arena/paddock, then add on bit by bit, rather than have all of it go up at once.
-For the number of stalls, consider this: Are you going to start stabling your horses every night, or just as needed? Myself, I don't like the idea of sticking a horse in a box all night; I'm only putting up two stalls and a large enclosed run-in. If a horse gets hurt or needs to be isolated, I'll put that horse in a stall. If I need to contain all of them, they'll get shut in the run-in area. This plan also lowers costs (both in construction and the long run), and means less work. Something to think about.
If you're planning on stabling all your horses, probably just put in six stalls, and leave room for expansion.
-Stall size: Average horses are usually comfortable in 12x12 foot stalls. Larger breeds in 14x14. Just figure in enough room for your largest horse to comfortably lay down.
-When planning where to put the barn, consider access to water and electricity, as well as what will have to be done to put a building/arenas there. I.e. Clearing forests/rocky terrain, leveling hills or building retaining walls, road access and room for a truck and trailer to turn around, water drainage, etc. Also, you'll need to consider a manure dump site... somewhere discreet, not only away from your home, but your neighbors' as well.
-I like sliding doors, myself, but I've heard complaints that they can be heavy and noisy. However, swinging doors can be kicked open more easily.
-If you look at companies such as Priefert, there are several modular stalls out there that can be bought and placed in your barn, with any option you could imagine. Even those windows.
-Make sure the main doors to the outside are big enough that you can walk out them with a horse on either side.
-For minimum ceiling height - 8ft to 11ft. Any lower than that and a rearing horse could hit it's head.
-I usually consider keeping feed and hay in a separate building to be a good idea... in the event of a barn fire, the first thing on your mind will be getting out the horses and you'll want your feed after the fire.
-For the main barn, consider having high ceilings, maybe even with fans in the roof peak. It helps with ventilation.
- If you go for an indoor arena, a possible, more cost effective option is to put up a ClearSpan building... this is basically a series of metal hoops with a strong fabric-type covering over it. It allows for natural lighting, and has less echoing.
Besides that, just pick up every horse magazine you can find and thumb through them for barn-related articles. Sometimes even an article about spring cleaning or barn improvements can be helpful. If you know any people who have a horse barn and/or arena, ask them what they like/dislike about their barn, and have a look around it if you can. You can pick up all sorts of helpful little things just by asking around.
Good luck. ^_^