I have been looking into a good carry gun quite a bit recently, and the Kahr PM9 is looking like it my be the one for me.
If you want to stick with the .45acp, check out the Kimber Carry model. It's still fairly big, but I think it's one of the smaller .45 options out there.
(edit) However, there's nothing like having a wheel gun as a backup and knowing that all you have to do is draw it and pull the trigger to make it go BANG. When I was younger, I used to have an old police S&W 38spl and really regret getting rid of it. It was one of (if not the most) accurate hand guns I have owned.