When I try to do this I get an null string for the width. From what element did you manage to get the width? Did you modify the default generated test page to test this?

 (From a default application auto-generated code)

public void Page_Loaded(object o, EventArgs e)
        {
           
            InitializeComponent();

        HtmlDocument _document = HtmlPage.Document;

         _document.GetElementByID("SilverlightControlHost").GetAttribute("id");  --> equal to SilverlightControlHost as expected

        _document.GetElementByID("SilverlightControlHost").GetAttribute("width"); -> Is null

... 

Did a few more, 

.GetAttribute("style") is set to [object],

_document.GetElementByID("SilverlightControlHost")..ccsClass is silverLightHost

But I didn't figure out how to extract the width from the style neither...

Any hints?

What I want to do at the end, is to make the page full screen on the browser (using BrowserHost.IsFullScreen = true;) then adjust the size of all the various components according to the available space on the screen...