Kivy understand self.pos and self.size

Whenever I try to span the canvas across the layout, all I have to do is this

<FloatLayout>:
    canvas:
        Color:
            rgba: 54 / 255, 60 / 255, 72 / 255, 1
        Rectangle:
            pos: self.pos
            size: self.size

I have been playing around and fail to understand what is self.pos referring to? By reading the documentation it seems the it points to current widget which should be Rectangle. However when I enter the debug mode, I notice that the default value of self.size is (100, 100).