FarrAway wrote an answer about Osprey Packs Atmos 50 Pack - 2800-3200 cu in on March 2, 2009
I'm also 5'4" with a small frame, but I was measured as a medium. I strongly recommend getting measured by someone who knows what they're doing. (I was measured twice - once by someone who did know what they were doing and once by someone who did not. The one who did not measured me as an XS, but the waistbelt of the XS pack was _completely_ above my hip bones.)I'd also recommend buying a first pack at a good outfitter's where you can try various packs on and walk around with weight in them.