One of my favorite keyboard shortcuts in Adobe Photoshop is using the Square Brackets keys (at times in conjunction with others) to control the size and hardness my brushes. It is a real time saver.
So, how do Square Brackets work in Photoshop:
1. Click on the the Brush Icon in your tool box.

2. Click the Right Square Bracket Key to increase the size of the brush OR Click the Left Square Bracket Key to decrease the size of the brush. Depending on the the current size of your brush the increments would be in either single, double or triple digits.
1. Click on the the Brush Icon in your tool box.

2. Click the Right Square Bracket Key to increase the hardness of the brush OR Click the Left Square Bracket Key to decrease the hardness of the brush.
Here is a sample layout of where these keys are: