Updated for 7.0
Click on the Settings icon (the gear) located to the right side of the search bar. This will take you to this page:
Within the horizontal navigation bar, select Users. Then, Costume Sizes. This will take you to this page:
Within the Costume Sizes box, select the yellow button titled +Add.
A new line item will appear that looks like this:
Type in the new costume size. Then, select the green button at the bottom right of the screen titled Update.