kv = """
FloatLayout:
    Widget:
        size: min(root.size), min(root.size)
        size_hint: None, None
        pos_hint: {'center_x': .5, 'center_y': .5}
        canvas:
            Color:
                rgb: 1, 0, 0
            Ellipse:    
                size: self.size
                pos: self.pos
"""
Builder.load_string(kv)


class MyApp(App):

    def build(self):
        return Builder.load_string(kv)
MyApp().run()
Центрирование объекта в Kivy