Your code behind executes at the server - so there is no screen.
So you'll have to do it client side and post it to the server, something like
<body>
<input type=hidden id=screenstuff>
<script>
screenstuff.value = screen.width + "_" + screen.height + "_" +
screen.colorDepth + "_" + document.body.clientWidth + "_"+
document.body.clientHeight;
<script>
These functions can change depending on browser ( I think, but don't quote me)
Then split around "_" at server
str = Request.Form["screenstuff"]
string[] s = str.Split('_');
for (int i =0; i<s.Length;i++)
{
Response.Write(s[i] + "<br>");
}