A longer jacket allows for coverage of your body when you are moving around a lot. Tell her that if you are skiing and fall with a shorter jacket on it will ride up on you and get snow all inside your business. You can demonstrate this with a normal sized t shirt. Just life your arms up in the air like they would if you were rag dolling. Honestly this is one of the primary reasons Saga was created. Taller people wanting jackets that fit well and functioned well.
You can also tell her baggier clothing allows for more freedom of movement. Maybe put on some tight jeans and try doing some calisthenics and show how restricted you are. Then throw on some sweatpants or mesh shorts and go to town.
In response to the "you can't see what the person is doing in the air" comment. She probably is thinking of someone competing in aerials or moguls where they critique every little body position. Try to explain to her that a huge part of what you are doing is being free to do tricks however you want. And creativity and individuality are rewarded not discouraged.