My mare does small hunter broodmare classes, and the height restrictions are over 148cm but under 158cm. Not sure whether it is the same for ridden or not.
Small hunter - definitely no bigger than 15.2hh. (We compete regularly in these classes). If it's any bigger than that, then it goes into weight classes - light, middle and heavy. I'm not sure about the rules now, but I believe that if you're doing Small Hunter Ridden Classes, then check to see if the rules are everyone needs a Life Height Certificate.