I wanted to reduce the size of the joystick.
I was able to do it by reducing the size of the two images Godot_TouchScreen_Joysticks_JoyBG.png and Godot_TouchScreen_Joysticks_Joy.png, and then adjusting their positions.
Is it possible to do this by changing properties (e.g. scaling) and not changing the images? I experimented with this, but without success.