The ideal size for a box stall might vary, depending on the size of the horses.
For ponies and compact breeds, such as Morgans or Arabians, a 10 feet by 10 feet space probably is sufficient, but for most horses over 15 hands, the accepted standard is 12 feet by 12 feet.
Perhaps 14 feet by 14 feet (the preferred size for Standard-bred breeding farms). If you plan to house foaling mares or stallions, you might want to have available at least a couple of stalls which are 16 feet by 16 feet. On the other hand, miniature horses are perfectly content in stalls only eight feet square.
I hope this helps :)