Indeed, this sounds silly, but because of a spelling mistake that is around for at least a year, and Microsoft is aware of that.
Take a look at Reflector :
The bit of code appends an invalid JavaScript command ("resizeable" instead of "resizable").
How to solve it ? ... wait for a patch..., or use the good old JavaScript, for example :
string javascriptOpenWindow = "window.open('" + link + "','MyWindow','toolbar=no,location=no,directories=no,status=yes,menubar=no,scrollbars=1,resizable=1,width=1024,height=800,left=20,top=20')";
System.Windows.Browser.HtmlPage.Window.Eval(javascriptOpenWindow);