In October, there were a lot of designs to choose from. They ranged from the pretty standard Mickey and Friends characters (e.g., Mickey, Minnie, Donald, Pluto, Goofy, etc.) to holiday-specific to some more unique characters (e.g., Figment). You can only select from the available choices (i.e., you cannot bring your own sketch, drawing or photo). You can add text, but you cannot have anything controversial printed; you also cannot print specific names of sports teams. I remember you could choose either a t-shirt (short sleeve) and long-sleeve shirt, but no sweatshirts.
The wait depends on how busy the shop is. When I've made shirts, the wait times ranged from 30 minutes to 1.5 hours. You can opt to use package pick-up to your resort (if you're staying at a Disney hotel). Otherwise, we found plenty to do and look out while we waited.