The stall width could be any less than 10' though, so it'd either have to be 10 or bumped up to 12.
I would bump it to 12' - The extra two feet really come in handy if you have to work in the stall, doctoring a horse or sedating one, etc.
Consider making a seperate room on one end of the barn, or one at each end that is a tack room, then put large cupboards with locks on them for each boarder. That way, you gain the space between stalls that you are allocating to a boarder and which will stand empty if there is no board horse in that stall. Most important though, that is way too serious a fire hazard. Aside from storing tack, boarders are going to be storing supplies to clean tack and horses, and heaven knows what else.