Go to Settings and Configurations at the top right of your screen. This will take you to this page:
Within the horizontal navigation bar, select Users. Then, Costume Sizes. This will take you to this page:
To edit the label of the costume, click the desired box to change the verbiage.
If you are wanting to remove a size, deselect the checkmark button located to the right of the size in question. To add the size back, select the checkmark.
To rearrange the order of the sizes, click the two arrows icon located to the right of the checkmark. Drag to the desired located.
When done, click the Update button in the bottom right-hand corner.
Removing costume sizes will lose any existing member records of that Costume size.