This is assuming that whatever you're selling the customer only wants one size, which would be the case for clothing but not for all things. One case would be

This is assuming that whatever you're selling the customer only wants one size, which would be the case for clothing but not for all things. One case would be lengths of cables or different weights of barbell plates, there the customer would want to buy several of different sizes. In that case all sizes should be visible with inputs for the desired quantities.