modal_scrollview.kv

#:import F kivy.factory.Factory

<TestView@ModalView>:
    size_hint: .5, .5
    ScrollView:
        size_hint: .9, .9
        pos_hint: {'center': (.5, .5)}
        GridLayout:
            cols: 10
            # this line is probably what you need
            size_hint: None, None
            size: self.minimum_size
            #hack don't do that
            on_parent:
                for i in range(1000): self.add_widget(F.Label(text='test', size_hint=(None, None)))

FloatLayout:
    Button:
        size_hint: None, None
        text: 'open me'
        on_press: F.TestView().open()
        pos_hint: {'center': (.5, .5)}