How to set widget size in percentage?

Question

Python

Good day, let's have a widget:
Python code:

class MyWidget(Widget):
    pass
 
class MainWidget(App):
 
    def build(self):
        main_widget = BoxLayout(orientation="horizontal")
        main_widget.add_widget(MyWidget())
        return main_widget

KV code:

<MyWidget>:
    canvas:
        Color:
            rgba: (0.3, 0.3, 0.3, 1)
        Rectangle:
            pos: self.pos
            size: self.size

And you need to make it so that it is not the full width of the window, but only half.
That is, something like self.size = 50%, 100%
I did not find anything about this in the documentation. Can
you please tell me how this can be implemented?

Answer 1

size_hint