Using a Wacom or Genius tablet

You can assign the pressure from your digitising pen to :

D - Depth, The harder you press the 'deeper' the brush (Z-)
H - Height, The harder you press the 'higher' the brush (Z+)
S - Size, The harder you press the bigger the brush.

Sensitivity slider.
0 - No sensitivity
128 - Half sensitivity
255 - Maximum sensitivity

The default sensitivity is 45