You can add custom details (ex: T-Shirt Size, Current Age, etc...) to get upon checkout when needed.
1. Edit the Checkout Item (Checkout --> View Checkout Items --> Edit the Item)
2. Scroll to the bottom of the page to Custom Fields.
3. Select the type: Text will give the user a textbox to type in and Dropdown will let them select something you add.
Note: When Selecting Dropdown each line inside the label will be an option for the user to select.
4. Add the field when you are done and you can sort them as needed.
5. Save the page and your custom fields should now be displayed on the checkout page.