It varies by manufacturer - if it's not on the tag, check the website; it will usually indicate the range by inches for each torso size. It will also include the hipbelt size in inches, and maybe information on how to swap hipbelts up or down in size (assuming the hipbelt isn't sewn on.)

If there is no local store nearby, and you don't want to drive to Dallas, most online retailers will let you accomplish the same thing as I described: just check their return policy before ordering. Most (including REI, which is really good) will let you return unused items, with the original tags still attached, for a full refund. So, order a couple of different sizes, or a couple of different styles, go through the process I suggested in your own living room (don't take the tags off the pack), maybe even take a walk around the block (not in the rain), and send back the packs you don't want in sellable condition for a full refund. You don't get the clerk's advice and help (which can be indispensable), but you save the drive time. Whether two-way shipping is cheaper than a round-trip drive is a determination you'll have to make.

If it were me, and this was my first pack, I'd make the drive.