Update! Click here (20 Brush and Painting Tool Shortcuts in Photoshop CC) to see twenty of my favorite tips related to brushes and painting in Photoshop CC in a single post.
With a painting tool selected, Control + Option (Mac) – drag left/right in order to decrease/ increase brush size. Control + Option (Mac) – drag up/down to decrease/ increase brush hardness.
On Windows, Alt + Right Mouse -drag left right to decrease/ increase brush size and up/down decrease/ increase brush hardness.
I prefer these shortcuts – even though they are a bit more complex than tapping the left or right bracket keys, as many international keyboards do not have brackets. Of course you can always customize the keys used to increase/decrease brush size and Hardness under Edit > Keyboard Shortcuts – choose “Shortcuts For : Tools”, scroll down (almost the bottom of the list) and enter any single character to set a new shortcut for Decreasing/Increasing Brush Size.