I’m 6’3 and just went through your exact dilemma. I stared at geometry charts for days, and when it comes down to it, you just need to ride em and see for yourself. If your inseam is ‘normal’ for 6-6’1, then you’re most likely an L in most brands. (For what it’s worth, I just bought a large Salsa hardtail)
General guide:
smaller=more maneuverable (some call this ‘fun’)
bigger= more stable
but I have a feeling you’re an L.