If I sell T-Shirts to a group of people, where all members have different shirt-sizes it would be helpful, if I could configure the design and when it comes to define the shirt-size I would have an individual Input for each size like so

S: | 3 | M: | 1 | L: | 2 | XL: | 0 | XXL: | 0 |

So in this case I have 5 Shirts with one design, ordered at once.

Obviously the total price of this order needs to be calculated according to the chosen quantity and shown right below the product before you add it to your cart.