main.py

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory


KV = '''
#:import Window kivy.core.window.Window

<SizeButton@Button>:
    target_size: 0, 0
    text: str(self.target_size)
    on_release: Window.size = self.target_size


BoxLayout:
    SizeButton:
        target_size: 640, 480
    SizeButton:
        target_size: 800, 600
    SizeButton:
        target_size: 1280, 720
    SizeButton:
        target_size: 1920, 1080
'''

class Application(App):
    def build(self):
        return Builder.load_string(KV)


if __name__ == "__main__":
    Application().run()