I just ordered one of these, here's some things that helped me:
5'5" wearing size L (he said it hits his knees)
6' wearing XL
quotes from different people:
"I'm 5'7 and I wear M.
The xavier jacket is huge, remember that."
"i'm 5'7 and i tried on a medium and it was ridiculously huge. "
"I tried an XL (I'm 5'9) and it came to my knees. "
"I'm 5'8 but i'm really skinny so everything falls longer on me. I got it in a size medium and it is plenty tall. "
zipper lengths:
M: 33.5"
L: 34.5"
XL: 35"