This trick is for Photoshop:

This works with any brush whether it be for painting, erasing, cloning etc...

With a brush active you can press the [ key multiple times to make smaller or the ] key to make the brush bigger.

Bonus Photoshop tip:
If you hold the Shift key down while following the instructions above it will adjust the hardness of the brush in 25% increments.

One click of the Shift+[ will cause the brush to have a hardness edge of 75% if the current setting is at 100%