I'm a female, 5'8" - not sure if I should...
I'm a female, 5'8" - not sure if I should go with size S/M or M/L; the capacity is the same, so what do the sizes represent?
The sizes represent torso length. To get yours, have a friend measure from the bumpy vertebrae on your neck (C7) to a line that's the same as the top of your hip bones (iliac crest). Take that number, then figure out your hip measurements, compare it to the pack sizing, there you go. My best guess is going to be the S/M is the size you'll need.
Here's a link for a YouTube video on how to do it.
Torso length doesn't seem to count for much in small frameless packs! Inevitably, they will be shorter than your torso!