For geometric similarity, which is a basic assumption for the power number, both the tooth height (h) and blade width (b) are some fraction of the impeller diameter. So, if h = 0.1 D and b = 0.2 D, or any other proportions of the impeller diameter, those factors become part of the power number and your expression involves impeller diameter (D) to the 5th power. Power number for any impeller is always some function of geometry factors such as blade width, height, angle, shape, etc.