Games that require you to spin the ---steaming pile of meadow muffin--- out of the ball (Golden Tee, bowling) use 3". Games that use the trackball as a controlled directional input device (centipede) use a 2.25". However, a 3" can always replace a 2.25" (see crystal castles). A 2.25" sucks as a replacement for a game that needs a 3" because it does not have the same spin momentum.