You can easily change your shirt size in your Account section at any time!
Log in to your account here https://catladybox.com/customer/account and click "Edit" under the active subscription you'd like to update.
You'll see a place where you can now select your box type, shirt size, and subscription term. You have to have each of these selected in order for it to let you save your change.
(You don't have to change the others, but just make sure you keep them selected as what they were before. If you want to change them, though, you can!)
Click Change, review your changes on the next screen, and then click Confirm.