Hi Brandon,
My husband is 6' 5", and wears size XXLT. While the selection is sometimes limited, I've always been able to find a golf shirt and/or t-shirt for him while at Disney World. I've often had good luck at The World of Disney Store, located at Downtown Disney. However, you can check in many of the gift shops in the Parks as well.
As far as the show
Fantasmic, you don't need reservations for that. You'll just need to plan on lining up at least an hour in advance to secure seats. Some guests do reserve the
Fantasmic Dining Package, which comes with vouchers for designated seating during the show. If you don't have Advanced Dining Reservations for that package, it is unlikely that you can walk-up and get it at one of the participating restaurants. This is because there are only a certain number available per night. Don't let that prevent you from watching the incredible show though. I recommend that you grab a bucket of pop-corn and stand in line, it is well worth the wait!
Hope this helps and that you have a wonderful vacation,
Kaylene