Use with Get_Property, only.
Returns the size (in pixels) of the client area of the specified control. Client area is all space occupied by control excluding border, caption bar, and scroll bars.
If you want to change the client size of the control, open the window in development mode and drag the control to the desired size.
clientareasize = Get_Property (objectname, "CLIENTSIZE")
Returns the size of the specified control in the format width:@FM:height