For me bigger is better:
Bigger ensures that you will almost always be able to fit everything in. Smaller means sometimes having to strap things to the outside of your pack.

Unless your pack fits properly, it won't feel right, even if it's supposed to be the Rolls Royce of backpacks. Follow the guidelines below to achieve pack-fitting Nirvana.

Universal Truth
How tall you are has no relation to what size pack suspension you will wear. It's the length of your torso that counts. Folks over 6 feet sometimes fit only into small suspensions. Conversely, folks 5 feet 6 inches tall can wear large suspensions.

Measuring Your Torso
You'll need a friend for this. First, find a soft tape measure (the kind a seamstress uses). Next, have your friend put a piece of masking tape on the seventh vertebrae ( the bony protrusion at the base of your neck between your shoulders). Now find the point on the small of your back that is exactly level with the top or shelf of your hipbones. You can help your friend here by sliding your hands down your side until they rest directly on top of your hips with your thumbs pointing inward. Tell your friend to be sure your thumbs are marking a somewhat straight line toward your spine and then have them place another piece of tape on this point. Using the soft tape measure, place one end on the seventh vertebrae and, following the contour of your spine, have your friend read the measurement where the tape measure touches the tape mark on your lower back. Write down the measurement.

What Torso Length Means To Pack Suspension Size:

If your torso length is less than 18 inches, you're most likely going to fit best into a small size suspension.
If your torso length is between 18 and 20 inches, you're most likely going to fit best into a medium suspension.
If your torso length is over 21 inches, you're most likely going to fit best into a large suspension.
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Other Fitting Basics

Hipbelts
Hipbelts should cup your hips and be sized so when you cinch them tight, there is still a bit of webbing left and the pads do not touch. Women with straight or narrow hips will fit fine in a standard waistbelt. Individuals with more curve to their hips will need a woman's style waistbelt.
Shoulder Straps
Shoulder straps should anchor to the backpack just below the seventh vertebrae and the crest of your shoulders. From the back, they should wrap comfortably, but securely around the shoulders with the bottom of the strap padding ending no closer than five inches below your armpit.
Sternum Straps
Sternum straps are meant to keep your shoulder straps from sliding off your shoulders under a load. They are not meant to support weight and should never be pulled tightly enough across the chest so as to restrict breathing.
Load Lifter Straps
Load-Lifter straps on the shoulders should form a 45 degree angle from the frame or top of the pack to a point on the shoulder strap at or above your clavicle (see, that anatomy class in High School is now paying off, isn't it!).