How to make Entry stretch across page in Grid?

Hello. I'm using the Grid to create a Login page. Below is what I'm starting with. The problem is that the Entry box doesn't stretch across the page and I can't figure out how to make it do so. It appears that the Grid takes up the whole page -- how can I make the Entry box take up 100% of the row? Also, almost all examples I find on your site are done in code - where can I find equivalent XAML samples? Thx

<Grid.RowDefinitions>


</Grid.RowDefinitions>

<Label Grid.Row="0" Text="Domain" VerticalOptions="Center" HorizontalOptions="Start" Font="25" />    
<Entry Grid.Row="1" />