JamesXI am 5'7" and got a medium 75L (wanted a 65L but black was sold out). My friend used to fit backpacks at REI and said that I should be good to with a medium. Here is Gregory's size chart: https://gregorypacks.com/gregory_fit_philosophy.html for reference.
JamesXSize of pack depends upon your torso length. I'm about 5'9" and require a large pack. However, a guy much taller (probably 6'2") was measured the same time I was for a medium. For best fit, get measured, either at an outfitter or on your own. If possible, have the outfitter load the pack and then try it out.
Spirox@T.Warren gives good advice below. If you don't want to measure - I have fairly normal proportions (vs extra long legs compared to torso or vice versa). I am 5'10", 195 and use a medium. If you are 5'8" or less and have "normal" proportions I would do a small. If you are 6'1" or above I would do a large.