Replying to What size tshirt should I get??!

Sup NS! I know this should be in gear talk, but I have to have an answer for my teacher in a few hours.
So quick question, in school we are tie dying some t-shirts, and they have shirts up to 6xl. These are regular cotton preshrunk tshirts, regular sizing.

I'm 6'4", and have a jacket with 37" zip length that I wish was like 38". I was going to get two, one to tie dye and one to leave blank using both to ski in wearing them either as their own, or over my jacket.

What size do you guys think I should get??
Thanks so much! And let thread die after today please :P