KivyMD's MDScreen Won't get sized to 100% of the windows size (uses the kv language) code example

Example: 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....