how should I determine size for this item?
You need to take a tape measure and determine your torso length. Once you have that, you can quickly determine the size you need.
My teenage daughter is 5'5" and her torso lenght was 17.75". Based on that, I bought her the Women's Small (S). It has been the best fitting pack she's ever had, and she loves it.
The manufacturer has an online sizing guide on their website that can be found on the top link returned after a quick search using 'gregory pack sizing chart'. Since I can't post the link in here, I'll copy the key portion for you here:
Measuring For Torso Length
Using a flexible tape measure or Gregory Fit-O-Matic, measure the length of your torso from the C7 vertebra (at the base of your neck) to the spot on the spine level with the top of the bony hip structure called the iliac crest. Do it two or three times to get an accurate measurement. If you are between frame sizes, always go with the smaller size.
XS: 14"-16" / 35-39 cm
S: 16"-18" / 40-44 cm
M: 18"-20" / 45-49 cm
L: 20"-22" / 50-54 cm
I am not totally sure about your question. I was measured at REI for this pack. I have a very short torso (14 inches) and should be in a xs. I am not really petite just average. But the guy we got was very good. He took other things into consideration and put me in a small. I am very happy with my pack. Especially after trying on other packs. My first hike in it was a 7 mile hike in. It was great. No soreness anywhere. The back is very adjustable.
SunnySports has very good prices! I needed a real person to try on a real pack so went to REI first!
My wife is 4'11" and the extra small fit her like a glove she carried 50lb with no problems for her first hike.we bough a north face small and she struggled with 35lb on a 3 mile test flat hike.
I am not a very experienced backpacker. This is the first time I have bought a backpack so this may or may not help you. I went to the Gregory site and it instructed me on how to measure my back for the correct size of backpack to buy. It gave a chart so you can know which pack to buy. Also, I have been told that you should not carry any more weight than what is one third of your weight. For example, if you are 120# you should only have 40# in your pack. This size is perfect for me. If it were bigger, I would put more in it and carry too much, yet, it is large enough for me to pack a lot in it. Love the Gregory's suspension and comfort!
The Torso Fit Range would be the best way to determine your pack size. Enjoy it! Zack.