The logitech mx1000 is a nice size. It's fairly large. The mx510 is also of a good size, but it's "egronomical" so it might not be egronomic for your hands. Go to BestBuy and try them out. Most mice run aorund the same size....some are small and others are regular. Then you have the slightly more puffy ones that have battteries on board i.e. the mx1000. A trackball would be the most comfortable but technically its a track ball and not a mouse.
I used to have the orginal Intellimouse explorer, that thing was big, and I loved it, but the scroll wheel finally gave out after 5 years of service.
I had trouble adjusting to my mx510 so I know how you feel.
The biggest one I've seen lately is the MX1000. If you haven't already, you should go take a look at it in store. It still isn't all that big, but it seems more so than the mx510.