I find smaller messenger bags harder to handle, they seem to flop around more, even with a cross strap.
I bought this sweet Low Alpine daypack. Plenty of (expandible)space, slim profile, stash pockets, great padding that doesn't get sweaty, a ***camelbak reservoir pocket*** etc. etc.
Anyway, if you're not tooo set on a messenger check it out.
http://www.campmor.com/webapp/wcs/st...uctId=38301982
http://www.lowealpine.com/Products/D...uctGroupID=615