Your height and weight are, to some extent, irrelevant. What is most important is your torso length. I've met 6'5" guys who use small/medium packs, and 5'8" guys who need a long.

Most pack websites (and online retailers) have instructions on how to measure your torso, and how that length relates to their size classifications. If you have no local store to visit, you can try that - though I've never been able to get a reliable self-measurement.

The best thing you can do is to take your gear to a store and get fitted for a pack, then load it with your own gear (sandbags do not give you a good approximation of weight distribution in a pack.) Walk around with it for a while. Try several packs until you get one that fits well. (Two points of etiquette: First, find out when the store isn't busy, and go then so you don't get in the way of other customers, and so you can have access to the clerk for an extended, uninterrupted period of time. Second, if you find a pack you like at the store, buy it there; don't try to save ten bucks by ordering it online - the store's help and availability are worth something.)

I've found that I prefer packs with adjustable suspensions; they let me fine-tune the torso length and mean a better chance that I'll find one that fits. (Osprey Kestrel and Deuter ACT packs have this feature, as do many others.)