It depends on so many things that it is almost impossible to say what size would be best.. It depends on the type of horse, how strong a rider you are, how strong a rider the horse needs, what you are going to be doing etc..
But like StormArion said, start looking from somewhere between 14.3-16.1hh and go from there, but don't be looking at the numbers.
I am 5'6 and weigh a bit less than 10 stones and have been riding all sizes and types, from ponies of less than 14hh to over 17hh horses, although I prefer horses around 15hh..
Nina x