Hi Denise,
I love my Arkel tailrider but I only use it on my Surly since my Orbea won't take a rack. Its a great bag but yes it is a little large. Hubby has the Delta post haste rack and bag and the bag has small panniers that unzip and hang down the side if you want more room. It attaches to your seatpost. I also have the piggy pack that I got from Adventure Cycling on my Orbea. It has a map case and is big enough for my small camera, inhalor, a Luna bar and my cell phone. An LBS can also add cage mounts for a second and even third bottle to your bike.
http://www.nashbar.com/profile.cfm?c...eid=&pagename=
http://www.adventurecycling.org/stor...fm?Category=28

The Delta is on sale at Nashbar right now for 49.00