win_resize.py

from kivy.app import App
from kivy.core.window import Window
from kivy.uix.widget import Widget


class ResizeApp(App):
    def build(self):
        Window.bind(on_resize=self.plop)
        return Widget()

    def plop(self, *args):
        print args

if __name__ == '__main__':
    ResizeApp().run()