[newbie] problem with geometry setting in Tkinter

[jenswaelkens at gmail.com]

I'm trying to set the geometry of my top window, but the size is
unaffected.
This is the code:

#!/usr/bin/env python
import Tkinter
top=Tkinter.Tk()
top.geometry=('900x460')
top.update_idletasks()
print (top.winfo_width())
print (top.winfo_height())
print (top.winfo_geometry())
top.mainloop()

and this is the result when running the code:
200
200
200x200+1+584


Can anyone here tell me what I am doing wrong and how to change it?
Thanks

Jens