Registration for the run Disney 2012 Inaugural Tinker Bell 1/2 Marathon opened to much fanfare last month (April 2011). Many of my friends and I were disappointed that only Women's sized long sleeve shirts were available to participants.
But now, Men's shirt sizes are listed as available in their new event FAQ section. (See Q: re: if Men are allowed to participate in the event.)
If you need a change, I would definitely contact them to update your shirt size. Because come Expo Friday and Saturday, no one wants a shirt that doesn't fit them, especially after paying over $120 plus pesky Active.com fees to register for the event.
I tried combing thru their website and came away with the email addie: email@example.com for those who want to update their proof of time for the event. If you need to update your shirt size, I would definitely email them to see if they are able to update your shirt size also.