The phrase "shirt size" refers to the specific measurements or dimensions used to determine the correct fit of a shirt on a person's body. Full definition
Email a list of everyone attending, including T shirt sizes, no later than 3 business days before your visit. (mostlymutts.org)
When choosing the size, we recommend you to use pregnancy t - shirt size as a model and always size down if you are between two sizes. (thankyourbaby.com)
Base size is based on shirt size and typically ranges from a 5 to a 7 for most people. (motherscircle.net)