The recommended code:
<GridLayout columns="2*,*" rows="*" width="100%" height="25%">
doesn't look as it should on my android.
First of all, the TextField and Button looked really thick. It actually did have almost 25% height of the entire screen.
Next, whenever the TextField is tapped, the keyboard is brought up and the height of the TextField and the button is halved.
A fix that worked for me was giving the height an absolute pixel value. eg.
<GridLayout columns="2*,*" rows="*" width="100%" height="400px">