To determine the size it depends what you want to carry in your handlebar bag. For my TransAm ride I started with a small Novara top bag and a front top bag on my surly nice rack, but later ditched the front rack and needed something bigger to hold my DSLR.

Ready for a price saver? I picked up a Schwinn handlebar bag en route at Wal-Mart in Wichita Kansas for about $15 complete with map sleeve - http://www.target.com/p/Schwinn-Hand...k/-/A-12309349

Worked great with a couple extra zip ties to remove any bounce and when it rained hard I simply moved my electronic valuables to my Ortliebs. Tons more pics of it on my site, best of luck in your choices!