Avalonia/samples/ControlCatalog/Pages/ViewboxPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.ViewboxPage">

    <Grid RowDefinitions="Auto,*,*">
        <StackPanel Orientation="Vertical" Spacing="4">
            <TextBlock Classes="h1">Viewbox</TextBlock>
            <TextBlock Classes="h2">A control used to scale single child.</TextBlock>
        </StackPanel>

        <Grid Grid.Row="1" ColumnDefinitions="*,Auto" HorizontalAlignment="Center" Margin="0,10,0,0">

          <Border HorizontalAlignment="Center" Grid.Column="0" BorderThickness="1" BorderBrush="Orange" Width="200" Height="200">
            <Border VerticalAlignment="Center" HorizontalAlignment="Center" Grid.Column="0" BorderThickness="1" BorderBrush="CornflowerBlue" Width="{Binding #WidthSlider.Value}" Height="{Binding #HeightSlider.Value}" >
              <Viewbox
                  Stretch="{Binding #StretchSelector.SelectedItem}"
                  StretchDirection="{Binding #StretchDirectionSelector.SelectedItem}">
                <Ellipse Width="50" Height="50" Fill="CornflowerBlue" />
              </Viewbox>
            </Border>
          </Border>

          <StackPanel HorizontalAlignment="Left" Orientation="Vertical" Grid.Column="1" Margin="8,0,0,0" Width="150">
            <TextBlock Text="Width" />
            <Slider Minimum="10" Maximum="200" Value="100" x:Name="WidthSlider" TickFrequency="25" TickPlacement="TopLeft" />
            <TextBlock Text="Height" />
            <Slider Minimum="10" Maximum="200" Value="100" x:Name="HeightSlider" TickFrequency="25" TickPlacement="TopLeft" />
            <TextBlock Text="Stretch" />
            <ComboBox x:Name="StretchSelector" HorizontalAlignment="Stretch" Margin="0,0,0,2" />
            <TextBlock Text="Stretch Direction" />
            <ComboBox x:Name="StretchDirectionSelector" HorizontalAlignment="Stretch" />
          </StackPanel>
        </Grid>

    </Grid>
</UserControl>