I'm 5'8 and the fit on the medium length wise is alright but ive got a big chest and shoulders which helps a lot in making sure tees aren't too long on me. If you're slim-ish and up to 5'11 then a medium would probably be cool. The large seems a lot bigger in both chest and length than the medium though so sizing up could be risky unless you do a lot of shrinking or like a pretty baggy fit.
I'm 5'11 and pretty skinny. I usually wear Large for the length but nowadays I'm looking for more fitted tees.
Could you do me a huge favor and post a pic of you wearing a medium and a large?