Ponies by definition are no more than 14.2 hands tall at the withers. A "hand" is 4 inches, so that's 58 inches or 4 feet 10 inches tall. Having said that, some pony breeds can grow bigger than that depending on genetics and diet. If a pony is taller, it cannot be shown in pony divisions. It wouldn't grow much taller than that, though, so if you're not into showing, don't worry about it. I would recommend getting a pony from someone who raises and trains ponies, so that your daughter can get some support in the beginning, from someone who knows the animal.