A long expression that defines the size of the control's thumb.
By default, the ThumbSize property is -1. If the ThumbSize
property is -1, the control automatically computes its size based on Maximum,
Minimum, LargeChange
and related properties. If the ThumbSize property is greater than 0, it
indicates in pixels the size of the thumb. Use the Orientation
property to specify whether the control is vertically or horizontally
oriented.