Hi Brian!

It is not possible to maximize a modal window,

but it is possible to simulate maximized size for modal XamWebDialogWindow.

<Grid x:Name="LayoutRoot">
        <ig:XamWebDialogWindow x:Name="DialogWindow2" IsModal="True" Left="50" Top="50" Width="300" Height="300">
            <Grid>
                <Canvas Background="Bisque" >
                    <Button TabIndex="0" Width="50" Height="20" Content="Button1" Canvas.Left="5" Canvas.Top="5" Click="Button_Click"></Button>
                </Canvas>
            </Grid>
</Grid>

private void Button_Click(object sender, RoutedEventArgs e)
{
 this.DialogWindow2.Left = 0;
 this.DialogWindow2.Top = 0;
 this.DialogWindow2.Width = this.LayoutRoot.ActualWidth;
 this.DialogWindow2.Height = this.LayoutRoot.ActualHeight;
}

I hope that can help.

Kind Regards!

Mihail