In Prestashop, we can set a default variant which will be showed on the frontend by default (at the first product showing). Then, each dimension drop list is filled with the dimension variant value (no null or empty value) and the customer is then forced to select a complete variant (no missing dimension).
The only problem for this, I think, is the case of one or more variants resulting of the variant values combinations don't exist. For example, I sell a clothe in 3 colors (black, white, blue) and 3 sizes (S, M, L) but the blue one exists only in L. So, selecting blue/S and blue/M must return a message telling that it doesn't exist (for example).
I use a lot this variant representation on my website (
www.delphine-lingerie.fr).
Regards,
Helgvor.