Always plan for expansion! When we opened our little 10 acre, 6 stall barn in the fall of 2008 to boarders we NEVER figured we would have grown to a 62 acre facility putting up a 72'x300' indoor arena/barn.
A bathroom is a must! We are just putting ours in now. For the last 2 years we've told boarders they can use the one in the house.. 1/2 of them took us up on the offer.. and although I didn't mind, it sucked having to keep my house spotless!
10 stalls is a good size especially if you will be cleaning those stalls by yourself which will happen even if you have hired help.