Basically, you want to take a flexible measuring tape and measure from the bump under your neck to the top of your hips. That should do it. If you know your dress shirt size or dress coat size, that can provide more information, but basically the length of your torso is what we need.
You can do a images.google.com search for a picture, try 'backpack sizing' and you'll get a bunch of pictures.
For the 75L, 60L and 45L packs the sizing is as follows:
Small fits from 14.5 to 17.5 inches
Medium fits from 17 to 19 inches
Large fits from 19 to 23 inches
For the 40L and 40B packs, sizing is as follows:
Small fits from 14 to 17.5 inches
Medium fits from 17 to 19.5 inches
Large fits from 19 to 22 inches
The 30L packs come in two sizes: Normal and Long.
Normal fits most folks up to about 19.5 inches.
Long fits the rest of us, from 19.5 to 24 inches.
The 20L packs are one size fits most.