I'm about 5'10 and I want it to be a bit...
I'm about 5'10 and I want it to be a bit baggy but not stupidly baggy should I get a medium or large?
I suggest a large. With layers, moving around on your board to strap in etc, a medium will be too small. When you first try it on, put on your snowboard pants too because if you try on the jacket with skinny jeans or something, it will look gigantic.
At 5'10 you are kind of teetering between a medium and a large. A medium should fit, however a large is going to be a safe bet. If you are layering and want a little extra room when you're spinning off a large cliff.