I've seen many advanced artists with big experience to open the drop down menu of the brush to change the size and softness of it when using the Brush, Clone stamp or Eraser tools. It's precise, but very time consuming and clumsy. There is a key shortcut which can help you almost simulate the dynamism of a tablet but using the regular mouse and keyboard.

To decrease or increase the brush size press "[" or "]". To change the softness of the brush press Shift+"[" or Shift+"]".