how to change window size in kivy python

#import everything you want
from kivy.core.window import Window #You must import this
Window.size = (600, 600) #Set it to a tuple with the (width, height) in Pixels
#(800, 600) is the default


#Your usual kivy code....

Here is what the above code is Doing:
1. Importing the Window class from the kivy.core.window module
2. Setting the Window.size to a tuple with the (width, height) in Pixels
3. (800, 600) is the default