You can buy a pen mouse on Amazon. A friend of mine has one and he swears by it, but I find it more difficult to use (or at least get used to) than a regular mouse for everyday things.
Actually my problem is not necessarily with the size, but more with the just reaching over and using a mouse. With the pen you either put it in the base or you leave it laying there. If I am working coding or something where I use the mouse rarely having to reach, pickup, use is more time consuming and frustrating to me. With a traditional mouse it is more natural just to move my hand to the right, use the mouse, and go back to typing away.