A lot of people seem to think its the size that matters; nay, I say to you: it's length that reigns supreme. And then, yeah, size.
Just to clarify, whether you're tall or short, male or female, you gotta measure your torso length. A lot of outdoorsy businesses will help you measure that if you're not sure but basically it's the bump at the back of your neck (no, not the tumor), the bony protrusion near the base of the back of your neck.
That's your last cervical bone. Measure from there until where your thumbs meet when you put your hands on your hips (typically on top of your iliac crests). I don't include weirdos who like to put their hands on the side all the way up, like under the breast or right against the armpit. It looks uncomfortable af. Stop it.
You could be a male with a short torso or a female with a long torso so make sure you get those measurements down for best comfort when hiking.