Four different people measured me in stores. Depending on who I asked, I was anywhere from 18-22 inches back size.

Followed the directions at McHale's website to measure my own size - turns out I am a firm 16.5" back size. This appears to be the "real" measurement as packs bought to this spec actually fit me without turning my back into a huge bruise or feeling off balance.
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"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

http://hikeandbackpack.com