How can I determine browser window size on server side C#

You can use Javascript to get the viewport width and height. Then pass the values back via a hidden form input or ajax.

At its simplest

var width = $(window).width();
var height = $(window).height();

Complete method using hidden form inputs

Assuming you have: JQuery framework.

First, add these hidden form inputs to store the width and height until postback.

<asp:HiddenField ID="width" runat="server" />
<asp:HiddenField ID="height" runat="server" />

Next we want to get the window (viewport) width and height. JQuery has two methods for this, aptly named width() and height().

Add the following code to your .aspx file within the head element.

<script type="text/javascript">
$(document).ready(function() {

    $("#width").val() = $(window).width();
    $("#height").val() = $(window).height();    

});
</script>

Result

This will result in the width and height of the browser window being available on postback. Just access the hidden form inputs like this:

var TheBrowserWidth = width.Value;
var TheBrowserHeight = height.Value;

This method provides the height and width upon postback, but not on the intial page load.

Note on UpdatePanels: If you are posting back via UpdatePanels, I believe the hidden inputs need to be within the UpdatePanel.

Alternatively you can post back the values via an ajax call. This is useful if you want to react to window resizing.

Update for jquery 3.1.1

I had to change the JavaScript to:

$("#width").val($(window).width());
$("#height").val($(window).height());

So here is how you will do it.

Write a javascript function which fires whenever the window is resized.

window.onresize = function(event) {
    var height=$(window).height();
    var width=$(window).width();
    $.ajax({
     url: "/getwindowsize.ashx",
     type: "POST",
     data : { Height: height, 
              Width:width, 
              selectedValue:selectedValue },
     contentType: "application/json; charset=utf-8",
     dataType: "json",
     success: function (response) { 
           // do stuff
     }

}

Codebehind of Handler:

 public class getwindowsize : IHttpHandler {

public void ProcessRequest (HttpContext context) {
    context.Response.ContentType = "application/json";
     string Height = context.Request.QueryString["Height"]; 
     string Width = context.Request.QueryString["Width"]; 
    }

I went with using the regex from detectmobilebrowser.com to check against the user-agent string. Even tho it says it was last updated in 2014 it was accurate on the devices I tested.

Here is the C# code I got from them at the time of submitting this answer:

<%@ Page Language="C#" %> 
<%@ Import Namespace="System.Text.RegularExpressions" %>
<%
    string u = Request.ServerVariables["HTTP_USER_AGENT"];
    Regex b = new Regex(@"(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce|xda|xiino", RegexOptions.IgnoreCase | RegexOptions.Multiline);
    Regex v = new Regex(@"1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-", RegexOptions.IgnoreCase | RegexOptions.Multiline);
    if ((b.IsMatch(u) || v.IsMatch(u.Substring(0, 4)))) {
        Response.Redirect("http://detectmobilebrowser.com/mobile");
    }
%>