Use the GtkWidget's allocation attribute, which contains a geometry tuple. Note that the widget must be shown (you may get around calling only realize on that specific widget when using libglade):

  >>> win = gtk.Window()
  >>> win.realize() # or win.show()
  >>> rect = win.allocation

Would return a gdk.Rectangle, which can be accessed by using:

  >>> rect.width, rect.height, rect.x and rect.y

or just convert it to a four sized tuple:

  >>> t = tuple(rect)

Meaning the widget is at the top left of the window, and is 158x22 pixels in size (the default size of a gtk.Entry, by the way)

To find out how high or wide a widget is (or would be) based on a string size and a font, see FAQ 5.16