center_y_problem.py

__author__ = 'Alan'
__version__ = '0.1'

import kivy
kivy.require("1.0.0")

from kivy.app import Builder
from kivy.app import App
from kivy.clock import Clock

mybutstr = """
#:import win kivy.core.window
Button:
    size_hint: None,None
    size: '65dp','35dp'
    center_y: win.Window.height*0.5
    center_x: win.Window.width*0.5
    text: 'test'
"""

class MyApp(App):
    def build(self, **kwargs):

        self.mybut = Builder.load_string(mybutstr)

        Clock.schedule_interval(self.resizeBut,0.1)

        return self.mybut

    def resizeBut(self, *args):
        self.mybut.height = self.mybut.height+10

MyApp().run()

Kovak's fix:
https://gist.github.com/Kovak/aac7ac890c6d5737f824