I have that trackball. As it is it's not very good (tons of friction), but if you remove the top cover and add a little 3-in-1 oil to the roller ends (there are no bearings in this one; the plastic rollers snap into plastic holders), it'll be usable as a mouse or for older games like Missile Command or Centipede. It backspins too much for games that require heavy spinning like Golden Tee or World Class Bowling.
The ball is about 2 inches in diameter. If I remember right, with the cover off I was able to replace the ball with a cue ball to get an arcade-size trackball, and the extra weight helped it spin a bit better. I ended up not using it because it was a bit too large for the space I had for it.
--Chris