A couple of magic keyboard shortcuts for Brush Tool in Photoshop for you:

• Use the bracket keys to change the size of the brush: [ to make it smaller and ] to make it bigger.
• You can also change the size of the brush by holding down Ctrl + Alt + Right Mouse button on Win / Ctrl + Alt on Mac and dragging left or right.
• Use Shift + bracket keys to change the hardness of the brush.
• Use number keys on the keyboard to change the Brush’s opacity, i.e. 1 for 10%, 3 for 30% or 4 and 5 for 45% (4 and 5 need to be typed quickly).

Enjoy! 😉