Joystick C# change touch zone?

Hi, I'm using the C# script for my joystick (from Unify Community: http://wiki.unity3d.com/index.php/Joystick), and it works fine. Getting input from the joystick also works, no trouble there. The only thing I'd like to do, is make the touchable zone of the joystick a little smaller. Is this possible with this script, and if so, how?

Thank you,

Arne Gevaert