Note: This is occurring due to the Fee Structure setup for the T-shirt Size options having an Additional Fee set to "0". If you wish the T-shirts to be free or a $0.00 amount, simply leave these fields blank.

Steps to resolve so the drop-down box is editable/selectable:

1. Go to Communities > Special Events > Friends Asking Friends > Kintera Thon
2. Highlight [Event Name], select Website Info
3. Click Fee Structure
4. Scroll down to the Registration Levels section and edit each Level to verify the following: 
    a. The Sphere Only checkbox is not marked
    b. The appropriate T-shirts checkboxes are marked, so they will display and/or be available for selection in the drop-down option box
    c. Set the Addtional Fee field to an appropriate amount, if you want this to be free, leave the field blank.
5. Click Save and continue for every Registration Level for this FAF Event.

At this point, you should be able to edit the T-Shirt Size through the back-end of the FAF event and/or the Participant should be able to edit their Participant HQ profile page to update the size.