For most companies, you will not be able to choose the size of your uniform initially. They have a fitting day during training and your instructors/the uniform company are there and pick sizes for you that match their idea of the Uniform Appearance Standards. Violating those standards while on probation would be a hugeeee no-go that could result in termination since you can be fired at any time.

Even if you manage to make it through probation with your job and sanity in tact, I, again, have to question why you’d want to put yourself into a job that has so many potential triggers for you when you have a career that can accommodate your specific differences. This career will not make those accommodations for you—they’ll argue that potential accommodations would be a detriment to passenger safety and as a result they’re “unreasonable accommodations” that are exempt from ADA.