Don`t use a widget`s `cget` method, use ``winfo` width|height .widget`
instead.  The `cget` method will only tell you what the widget wanted to
be or what you told it to be with `.widget configure -width|height ...`.
If the packer resizes it to fill the slave space, or the user resizes
it, then only `winfo` will return the actual new size.  To summarise:

 cget -width
 cget -height

These give you back the size that you asked for in units specific to the
widget, e.g. it could be pixels, lines, characters, miles etc. This size
is used by the widget to calculate the initial size in pixels of the
whole widget (including borders etc.). The meaning of these options is
totally widget specific.

winfo reqwidth 
winfo reqheight 

This gives you the size in pixels that the widget has calculated is
optimal given all of the options set on that widget. This information is
passed to the geometry manager which decides what the actual size of the
widget should be.

winfo width 
winfo height 

These give you the actual size of the widget as set by the geometry
manager. This may be larger or smaller then the requested size.