A quick and easy way to adjust your brush size
If you want to adjust your brush size in Photoshop you can do that with the slider in the options bar. But the problem is that you do not see the effect while you're adjusting.
So you'll find your self constantly moving your mouse (with the brush tool) from where you're working in your image to the option bar and vice versa.
Another way to adjust your brush size is to leave your cursor where it is and right-click in the image. A panel appears where you can adjust the size of your brush as well as the hardness.
This way your cursor is still near the position where you need it.
Using Keyboard shortcuts to adjust your brush size and brush hardness
An other way to change your brush size is the use of keyboard shortcuts:
Use the bracket keys ( ']' and '[' ), so you can immediately see the effect.
[ to decrease and
] to increase
This works for several tools, like the Quick Selection Tool, the History Brush, the Clone Stamp, the Pencil Tool and the Dodge -, Burn - and Sponge tool.
This also works in Lightroom.
To adjust the hardness use the same keys while holding down the Shift key.
So if you want to know some more about keyboard shortcuts, please read the last part of the post Keyboard shortcuts for zooming.