You need to use the Move method of the Form object. Here's an example that reduces both the height and the width of a form by 2, without actually moving it (i.e. its left corner does not move on the screen):
Private Sub Command_Shrink_Click()
Me.Move Me.WindowLeft, Me.WindowTop, Me.WindowWidth / 2, Me.WindowHeight / 2
If you specify absolute values as arguments, do not forget that they must be expressed in twips (there are 1440 twips in an inch, 567 twips in a centimetre).