NANCY A I'm sorry, but since I have the medium and have never tried a small, I don't know what the difference is. I'm 5'4" and the medium feels perfect to me. (I use it for easy to moderate day hikes in the Colorado Rockies.) I will say, though, that it's not great for use with a hydration reservoir because the reservoir eats up a lot of space, reducing gear space considerably. Even so, it's an extremely comfortable pack and I will, no doubt, get a lot of use out of it.

HOLLY W My understanding is torso length. I am 5'1", so I went for the small.

AMY H Size has to do with the torso lenght of the pack. Small size is a bit less in storage volume also. Gregory's website under specs has the deminsions and weight of each size. I am 5 ft 6 and use the medium. On Greggory Pack's website under specs they have the torso sizing info and a downloadable .pdf on how to figure out the proper size.

ALEX M I bought one for my wife. She is medium built and her height is 5.'6.5 and her weight is 146. I bought it medium and it fit perfectly. She loves it and is very happy with it.