To change the reference size buyers see on your listings, start a new listing or edit any existing item. Tap Sizing Notes next to Size, then tap Edit under “Show my size to help buyers” and update your size. This is a profile-wide setting, so you only need to change it once.

When shopping, you'll see a floating “My Size” tab at the bottom of the screen. Just tap that to update.

You can also tap your profile photo at the top right of your app, then tap on ‘Account & Settings’. You'll see the option to update your ‘My Size’ tab from there.