これは、Kivy の Window モジュールを使用して ScrollView のサイズをウィンドウのサイズにバインドすることで実現できます。以下に、ScrollView の高さと幅をウィンドウの高さと幅の 80% に設定するコード例を示します：

```python
from kivy.app import App
from kivy.core.window import Window
from kivy.uix.scrollview import ScrollView
from kivy.uix.label import Label


class MyScrollView(ScrollView):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.size_hint = (None, None)  # disable size_hint to allow fixed size
        self.bind(size=self.set_size)  # bind the size to the window size
        self.set_size(Window, Window.size)

    def set_size(self, instance, size):
        self.height = size[1] * 0.8  # set the height to 80% of the window's height
        self.width = size[0] * 0.8  # set the width to 80% of the window's width


class MyApp(App):
    def build(self):
        return MyScrollView(
            size_hint=(None, None),
            pos_hint={'center_x': 0.5, 'center_y': 0.5},
            do_scroll_x=False,
            Label(text='This is a scrollable view'),
        )


if __name__ == '__main__':
    MyApp().run()
```

この例では、MyScrollView クラスは ScrollView を継承し、__init__ メソッドをオーバーライドして size_hint プロパティを無効にし、size プロパティを set_size メソッドにバインドしています。set_size メソッドは ScrollView の高さと幅をそれぞれウィンドウの高さと幅の 80% に設定します。最後に、MyApp クラスは MyScrollView ウィジェットの size_hint プロパティと pos_hint プロパティを設定し、その子として Label を追加します。
You can achieve this by binding the size of the ScrollView to the size of the window using Kivy's Window module. Here's an example code snippet that sets the height and width of the ScrollView to 80% of the window's height and width:

```python
from kivy.app import App
from kivy.core.window import Window
from kivy.uix.scrollview import ScrollView
from kivy.uix.label import Label


class MyScrollView(ScrollView):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.size_hint = (None, None)  # disable size_hint to allow fixed size
        self.bind(size=self.set_size)  # bind the size to the window size
        self.set_size(Window, Window.size)

    def set_size(self, instance, size):
        self.height = size[1] * 0.8  # set the height to 80% of the window's height
        self.width = size[0] * 0.8  # set the width to 80% of the window's width


class MyApp(App):
    def build(self):
        return MyScrollView(
            size_hint=(None, None),
            pos_hint={'center_x': 0.5, 'center_y': 0.5},
            do_scroll_x=False,
            Label(text='This is a scrollable view'),
        )


if __name__ == '__main__':
    MyApp().run()
```

In this example, the MyScrollView class inherits from ScrollView and overrides its __init__ method to disable the size_hint property and bind the size property to the set_size method. The set_size method sets the height and width of the ScrollView to 80% of the window's height and width, respectively. Finally, the MyApp class sets the size_hint and pos_hint properties of the MyScrollView widget and adds a Label as its child.