With the exception of the Warp brush, you can quickly adjust the size of any raster brush (including the Clone brush, the Eraser and the Background Eraser) as well as the Art Media tools without taking the cursor off the canvas and into the Tool Options palette. To adjust the brush size via the keyboard:
Press Alt + X to decrease the brush size by one pixel.
Press Alt + C to increase the brush size by one pixel.
Hold down Alt and Shift and press X to decrease the brush size by 20 pixels.
Hold down Alt and Shift and press C to increase the brush size by 20 pixels..
Hold down the Alt key and drag (to drag, hold down the left mouse button and then move the mouse) the left mouse button up or down to rapidly change the brush size.
Hold down the Alt and Shift keys and drag the left mouse button; the size will be set to its lowest value (3 pixels for the Art Media tools, and 1 pixel for the raster painting tools) and then you can drag to increase it from there.