Keyboard shortcuts for brush size?

Hi,

Has anyone found a keyboard shortcut to change a brush size? I know you can use the scroll wheel on a mouse but I use a Logitech MarbleMouse which is a trackball (upside down mouse) and doesn't feature a scroll.

I think in Photoshop you use the square bracket keys to change the size of a brush and something similar would be useful in Aperture. Is there a way I can assign brush increase/decrease to some keys?

Cheers,
Dave.