Q: How can you control the size and hardness of the Brush tool in Photoshop with keyboard shortcuts?
A. With the Brush tool in Photoshop Selected, you can make your brush size smaller or larger by pressing the left ([) and right bracket keys (]). Make your Brush softer or harder by pressing Shift+Left bracket, or Shift+Right Bracket.
Keep in mind that if you are in Precision mode (Caps Lock key is on) you will not see the change in the brush tool cursor.