By default, GtkWindow objects are displayed as compact as possible. In order to specify the preferred window dimensions, first set the visibility of the window to “No” in Glade Interface Designer (properties -> common tab.)

In the code of the window class, call set_size_request(x,y) before making the window visible. E.g.:

class appgui:
  def __init__(self):
    gladefile="gui.glade"
    windowname="window1"
    self.wTree=gtk.glade.XML (gladefile,windowname)
    ...
    ...
    ...
    self.window = self.wTree.get_widget("window1")
    self.window.set_size_request(320,640)
    self.window.show()
    return

The basics of creating a GUI using PyGTK and Glade:
http://www.learningpython.com/2006/05/07/creating-a-gui-using-pygtk-and-glade/