Wood and shingle roofed buildings are hotter as they absorb heat. Plan on vents for shingle roof units. In the south steel is a better buy. This was chosen to accommodate five foot high shelf units commonly sold at lumber yards, as these are less expensive and more flexible in use than built in shelving. This size for backyard buildings looks the best with out appearing top heavy.
Taller units are available.
We recommend you consider more square footage before going taller. Leveled on blocks is all that is needed, tie downs are needed in high wind area. A wood barn style with 4' side walls will cost the least. It is also the worst buy. The side walls are to low to be of much use, that leaves the back wall for tall items and the floor.