How big does horse stalls have to be ?

hi , I have got a wee 13.2hh pony an I have a stable which is approx. 9ft X 8ft i think, something like that anyway... is that a good size? Also I am hoping to get a 16.2hh mare with a month-old foal, so how big would the stall need to be? I measured out the 2 different spaces we have and the first one was approx. 18ft/19ft X 10ft and the second one was approx. 18ft/19ft X 9ft, would they be big enough ? if it is , I shall be building both of them for when the foal matures and becomes bigger , which wont be long. Thanks :) by the way if you have any tips I'd be happy enough to hear them. xo