When you create a competition, custom field T-Shirt Size is included by default - an athlete will automatically be shown a drop-down menu for t-shirt size when they register for your competition. Default size ranges included are Ladies' S-XL and Mens' S-XXL. If your competition does not offer t-shirts, you must delete this field to avoid athlete confusion.
You don't need to do anything else other than setting up and publishing your competition to collect athlete shirt sizes. We take care of it for you!
If you want to edit the sizes we offer, customize the custom field further, or delete the field, check out this article: Add custom fields to registration form.