By default, when a competition is created, we create a default custom field to collect t-shirt sizes for athletes at registration. If this field is unchanged when you publish your event, an athlete will automatically be shown a drop-down menu for t-shirt size when they register for your competition.
You don't need to do anything else other than setup and publish 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
But how do I export the t-shirt data? Look at this article: Exporting athlete registration information