I know horses are measured in hands, but how big is one hand in cm/inches ????
The old way of finding the ideal size is a horse that comes between your chin and your nose at the wither and is of a similar build to you. For someone 5'6 "that would be between 15h and 15.3h. Of course it depends entirely on your experience and confidence though. Something smaller may be better for a beginner to gain both and a confident, skilled rider can control a much bigger horse.
Hope this helps.
One hand is four inches. A 15 hand horse is 60 inches tall. For a pony to show it has to be under 14 '2 "hands.
4 inches
4 inches