If you scroll over the admin tab and then click on Network settings. Once it opens click on the "registration Tab"
Scroll down and you will see a section for "Custom Registration Questions" - click on Add question and you can ask for the shirt size information. It has worked out really well for us especially during championship nights because we can get a better estimate of what sizes of shirts we need to give out to our champs. Instead of taking several of each size we take the highest number of each size from each team. That way we have enough for championship night. Hope it helps. Oh yeah, also next time the participant logs on to imleagues it will prompt them to add any new custom question you might have added to the registration.