Maybe instead of Category by type (Fantasy, Sci-fi, etc.), maybe categories by size. Like a 30-50mm base sizes, 50-300 mm, and 300+ for the big pieces. Then break down each size category by type (urban, landscapes, abstract, whatever). This way you wouldn't have to compare, say, a cleverly done base sized for a single mini and something the size of a gameboard. Maybe even a seperate "Hirst Artistry" category for those into making pieces with that.
Just my thoughts on it.....
*EDIT* Sheesh, three more posts popped in here since I began writing this one, I see BK had the same idea as me, and a bit faster to write it *