Set or get the number of visible entries and update the scrollbar's thumb size
accordingly. If you haven't provided a target widget by specifying Scrollbar.Target,
you have to update the scrollbar's thumb size using this attribute whenever the size of the
target widget changes.
Note that unless Scrollbar.AutoScale is to False, this value will be in
device-independent pixels. If you want to have physical pixels instead,
you must set Scrollbar.AutoScale to False.