South St Track Pants

How does the sizing work? Because I can’t select one size it just shows all 3 when I check out and I’d ideally like a medium.

Hey there, I’d ideally like a size medium, but when I go to check out it lists all the sizes from Small extra small and medium, and I don’t know which one I’m going to get or how to select which one I want. Could you please help with this?