Thanks for the advice. I see now that the backpacks I'm looking at are
not external frame, they are just cheap backpacks with those aluminum
stays (which I found out today can be removed.) I'm not looking at
$300 backpacks, I'm looking in discount stores at $12 to $20 backpacks
(of course, I'm living in a third world country, so these packs are
probably the same as $50 backpacks you might find in Wal-Mart or
wherever)

I'm not sure how many liters I'm looking for, I just kind of look at
the backpack and ask myself if it's about the size I need.

So what is the advantage of these aluminum stays? I could get a cheap
pack with them or without them.

Thanks everyone.