from tkinter import *
... program ...
frame = Frame(width=640,height=480)
frame.pack()
...program...
#now to change the frame size
frame.configure(width=800)

doesn`t work ...
I tryed from.config() too but it doesnt work

I can not reproduce your problem:

try:
    from tkinter import *
except ImportError:
    from Tkinter import *

root = Tk()

frame = Frame(root, width=640, height=480)
frame.pack()
#now to change the frame size
frame.configure(width=200)

mainloop()