屏幕顶部的Instagram工具栏

在这里,我试图在屏幕顶部创建一个简单的Instagram工具栏,这是我的py文件:

from kivymd.app import  MDApp
from kivymd.uix.label import MDLabel
from kivy.core.window import Window
from kivymd.uix.screen import MDScreen
from kivy.lang import Builder
class HomePage(MDScreen):
    pass
class InstagramApp(MDApp):
    def build(self):
        Window.size =[300,600]
        Builder.load_file("home_page.kv")
        return HomePage()

InstagramApp().run()

以及相应的kv文件

<HomePage>:
    orientation: 'vertical'
    # App Bar
    MDBoxLayout:
        padding: '5dp'
        adaptive_height: True

        MDIconButton:
            icon: 'plus'

        MDLabel:
            text: 'Instagram'
            halign: 'center'
            valign: 'top'
            font_size: 25


        MDIconButton:
            icon: 'facebook-messenger'

但结果是:

我怎样才能把它移到最大号?提前感谢