I have two items a shirt and a jersey, one has sizes as an option and the other does not.
This is related configuring your premium items while under the Registration Upsells Tab in the Teamraiser list. While editing the premium item, and going to step 2: Configure Upsell Options, then go to Step 5: It says:
Display Information: Determines how his Upsell will appear on registration forms (cannot be changed once saved)
This is very important when setting the items up, to double check your settings are correct. Once the product is saved, it will need to be archived/deleted and recreated for these options to be changed to something different. To ensure that options are available for a product, make sure to "Select List" from the dropdown, and not checkbox, which just indicates that it is to be included.