Filling Browser Window with Silverlight Application

First, I don't set the height/width in the user control. Instead, I set the DesignHeight and DesignWidth (in the "http://schemas.microsoft.com/expression/blend/2008" namespace) and I set the alignment to stretch

<UserControl ... 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
    HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
    d:DesignHeight="1050" d:DesignWidth="1680">

In my HTML, I set the Height and Width to 100% like this...

<div style="height: 100%; width: 100%; position: fixed;">
        <asp:Silverlight runat="server" Source="~/ClientBin/My.xap" ID="MyId" 
            Width="100%" Height="100%" />
</div>

At that point, everything works for me to have it take the entire window.