T-shirt sizeing is a relative sizing unit that uses T-shirt sizes that are relative to one another such as XS, S (Small), M (Medium), L (Large), XL, etc.
The agile team decides how a small (S) user story look like. Then other user stories are sized relative to the agreed small story. If a User Story is slightly smaller or larger, it will be sized XS or M or L. If the difference is very large, Agile team will choose extra sizes like XXS, XS, XL and XXL.