insert background image in a grid, that fits the size with Xamarin forms pcl

Question

I have the need to insert a background in a grid that is suitable alone according to the device size. I tried a thousand combinations but remains small, or does not fit all. In the picture below you can see circled in red the image that I have to adapt to the Grid, Grid which is in the second half of the bottom screen.

I'm trying to put the image in the grid, the image has x: Name = "backgroundImage", what is the object that I have to adapt to the entire grid of the second half of the screen, how can I do? here's the code:

 <?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="Fimap.Login"
             BackgroundColor="#333">
  <ContentPage.Padding>
  <OnPlatform x:TypeArguments="Thickness"
                iOS="0,0,0,0"
                Android="0,0,0,0"
                WinPhone="0,0,0,0" />
  </ContentPage.Padding>
  <ContentPage.Content>
    <ScrollView>
      <StackLayout VerticalOptions="Center" HorizontalOptions="CenterAndExpand" Orientation="Vertical" Spacing="0">
        <ActivityIndicator x:Name="loadingBeforeLogin" IsVisible="true" Color="#008ECC" IsRunning="true" />
      </StackLayout>
      <RelativeLayout VerticalOptions="Center" HorizontalOptions="CenterAndExpand" x:Name="allContent">
        <Grid>
          <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"></ColumnDefinition>
          </Grid.ColumnDefinitions>
          <Grid.RowDefinitions>
            <RowDefinition Height="5*"></RowDefinition>
            <RowDefinition Height="5*"></RowDefinition>
          </Grid.RowDefinitions>
          <!-- immagini loghi e scritta login sopra-->
          <Grid Grid.Row="0" BackgroundColor="#1f2429">
            <Grid.ColumnDefinitions>
              <ColumnDefinition Width="2*"></ColumnDefinition>
              <ColumnDefinition Width="6*"></ColumnDefinition>
              <ColumnDefinition Width="2*"></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions Height="*">
              <RowDefinition Height="2*"></RowDefinition>
              <RowDefinition Height="6*"></RowDefinition>
            </Grid.RowDefinitions>
            <Image Aspect="AspectFit" x:Name="logoScritta" Grid.Row="0" Grid.Column="1" Source="Images/fimaLogoLogin.png"></Image>
            <Image Aspect="AspectFit" x:Name="logo" Grid.Row="1" Grid.Column="1" Source="Images/logo.png"></Image>
            <Label TextColor="#fff" Grid.Row="2" FontSize="22" Grid.Column="1" Text="Login" />
          </Grid>
          <!-- username e password input e ricorda password -->            
          <Grid Grid.Row="1">
            <Image x:Name="backgroundImage"/>
            <Grid.ColumnDefinitions>
              <ColumnDefinition Width="2*"></ColumnDefinition>
              <ColumnDefinition Width="6*"></ColumnDefinition>
              <ColumnDefinition Width="2*"></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
              <RowDefinition Height="1*"></RowDefinition>
              <RowDefinition Height="2*"></RowDefinition>
              <RowDefinition Height="2*"></RowDefinition>
              <RowDefinition Height="2*"></RowDefinition>
              <RowDefinition Height="2*"></RowDefinition>
              <RowDefinition Height="2*"></RowDefinition>
            </Grid.RowDefinitions>
            <ActivityIndicator x:Name="loading" Grid.Row="0" Grid.Column="1" IsVisible="false" Color="#008ECC" IsRunning="true" />
            <Label TextColor="#2196F3" Grid.Row="1" Grid.Column="1" Text="" />
            <Entry TextColor="#2196F3" PlaceholderColor="#A9D6FA" FontSize="24" Grid.Row="1" Grid.Column="1" Placeholder="Username" x:Name="UsernameEntry" Text="" />
            <Label TextColor="#2196F3" Grid.Row="2" Grid.Column="1" Text="" />
            <Entry TextColor="#2196F3" PlaceholderColor="#A9D6FA" FontSize="24" Grid.Row="2" Grid.Column="1" Placeholder="Password" IsPassword="True" x:Name="PasswordEntry" Text="" />
            <Button x:Name="LoginButton" FontSize="22" Grid.Row="3" Grid.Column="1" Text="Accedi" Clicked="Login_OnClicked"/>
            <Grid Grid.Row="4" Grid.Column="1">
              <Grid.RowDefinitions>
                <RowDefinition Height="1*"></RowDefinition>
              </Grid.RowDefinitions>
              <Grid.ColumnDefinitions>
                <ColumnDefinition Width="7*"></ColumnDefinition>
                <ColumnDefinition Width="3*"></ColumnDefinition>
              </Grid.ColumnDefinitions>
              <Label TextColor="#2196F3" Text="Ricorda accesso" Grid.Row="0" Grid.Column="0"></Label>
              <Switch x:Name="switchRememberPassword" Grid.Row="0" Grid.Column="1"></Switch>
            </Grid>
            <Label x:Name="recuperaPassword" Grid.Row="5" Grid.Column="1" TextColor="#2196F3" Text="Hai dimenticato la password?" FontSize="12"></Label>
          </Grid>
        </Grid>
      </RelativeLayout>
    </ScrollView>
  </ContentPage.Content>
</ContentPage>

SOLUTION add image from codebehind c# into RelativeLayout with name relative

Image backgroundImage = new Image();    
backgroundImage.Source = "loginBackground.png";
                backgroundImage.Aspect = Aspect.AspectFill;            

                relative.Children.Add(backgroundImage,
                    Constraint.RelativeToParent((parent) =>
                    {
                        backgroundImage.WidthRequest = parent.Width;
                        return 0;
                    }),
                    Constraint.RelativeToParent((parent) =>
                    {
                        return parent.Height - backgroundImage.Height;
                    })
                );

Answer 1

In iOS project before calling the App constructor:

App.ScreenSize = new Size(UIScreen.MainScreen.Bounds.Width, UIScreen.MainScreen.Bounds.Height);

In App class in PCL:

public static Size ScreenSize;

And simply when giving the size of grid columns and rows you should use it something like:

App.ScreenSize.Width * 1, App.ScreenSize.Height * 1