There is a quick way to figure out if a pack will fit you properly or not: Just take your back and hips measurements and match them to pack size specifications. Your back length measurement is the most important. 

Sizing the Harness:

How to take your back measurements:

• Measure your torso length: Locate the tops of your hipbones.

• Draw an imaginary line between them across your back. From the point where the line crosses your spine, measure up to your seventh cervical vertebra. 

• To locate the C7, tilt your head forward; the C7 is the biggest knob along the spine.

• Check pack's specs for correct size!

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Sizing the Hipbelt:

How to take your hips  measurements:

• Measure around your hips, 2" (5.2cm) below the Hipbones

• Check pack's specs for correct size!

• Most simpler pack will not specify hipbelt size which is one size. 

Note:

This page will help you tell which pack size will technically fit you. But it can't predict  personal preferences: different people wear their pack differently (e.g., high on the hips, or low on the hips).  Ultimately, actually trying on an appropriately loaded pack at home is really the only way to tell what works for you.