ScrollViewer Auto Sizing

Below is the XAML for a portion of the main page of my WP7 app. Note that the height of the ScrollViewer is explicitly set.

        <Canvas Height="224" HorizontalAlignment="Left" Margin="12,24,0,0" Name="CalculatorScreenCanvas" VerticalAlignment="Top" Width="450" Grid.Row="1">
            <TextBlock Canvas.Left="13" Canvas.Top="99" FontSize="25" Height="30" Name="DataInputShadow" Text="2" Width="295" Visibility="Collapsed" />
            <ScrollViewer Canvas.Left="0" Canvas.Top="29" Height="176" Name="CalculatorScreenScrollViewer" Width="456" VerticalContentAlignment="Stretch" VerticalScrollBarVisibility="Visible">
                <Canvas Height="108" Name="CalculatorScreenSubCanvas" Width="455" VerticalAlignment="Stretch">
                    <TextBlock Canvas.Left="10" Canvas.Top="-34" FontSize="25" Height="181" Name="DataInput" Text="tan 25 + cos 45 + 4 cos 46" TextWrapping="Wrap" Width="439" VerticalAlignment="Stretch" />
                    <TextBlock Canvas.Left="10" Canvas.Top="10" FontSize="25" Height="38" Name="CalcCursor" Text="|" Width="8" />
                </Canvas>
            </ScrollViewer>
        </Canvas>