I should have thought of that myself. I guess I thought it was something that was common knowledge and I'd look like an idiot asking the question.

The pack is a Gossamer Gear pack (a G4) so I called their number and the guy explained to me that the torso measurement is from the shoulder strap attachment down to "more or less" where the hip belt goes.

I noticed also while checking that Gossamer Gear is getting ready to upgrade their entire line of packs so they are selling the current style at discount prices. I may just get one of those rather than messing with this eBay thing (especially if the size turns out to be wrong).