Why do we impose a rigid structure on your inventory?
Category 
Product Family
Product Line
Item
We do this so that you can automatically start selling your products online. In order to offer online booking you need to conform to this structure so your products can be presented in an orderly manner, where images, descriptions, prices and sizes are all arranged so that your customers can find the right product:
From an online point of view you have to think about :
Categories - a logical grouping of similar products
Product Families (PF) - an interchangeable product. Something that will share the same image, notes and price.
Underneath each PF you have your sizes.