Yes, you can add the size of the t-shirts with the Name & numbers. Here is how you can do it in your store.
Login to Backend
Navigate to the Stores ➤ Configurations ➤ Product Designer Settings, you will land on the Configurations page.
Now scroll down to the Attribute Code under the Name Number Import section.
In the Attribute Code field, select the Size option and click on the Save Config button to save the configurations.