Hi there, @wpbetsy,
When ordering variations, it’s important to remember that there are two separate instances of where these can be ordered: one for the frontend, and one for the product admin area.
Product Admin Area: This one is aimed at establishing a custom order in the admin area only. This custom order will have no impact on the frontend.
Frontend: This is done on the same page where you’ve first created the attribute. Essentially, you’ll want to head on over to Products > Attributes > Product Size > Configure Terms and then drag & drop the terms into your desired order. Here are some screenshots of that in action:
https://cld.wthms.co/ghNhW7
https://cld.wthms.co/stEpOv
Best,
Ross