You may be looking for the get_allocation method:
The get_allocation() method returns a
gtk.gdk.Rectangle containing the
bounds of the widget's allocation.
or size_request:
The size_request() method returns the
preferred size of a widget as a tuple
containing its required width and
height. This method is typically used
when implementing a gtk.Container
subclass to arrange the container's
child widgets and decide what size
allocations to give them with the
size_allocate() method. Obtaining a
size request requires that the widget
be associated with a screen, because
font information may be needed.
Also remember that the size request is
not necessarily the size a widget will
actually be allocated.
The latter is what the widget would like to have as its size (given font information), the former what it's actually given by its container when it lays out the widgets.