Code:
 
 if (fileUpload.PostedFile.ContentLength > 409600) 
{
 // poruka o gresci ... 
}

Code:
 

   protected void Application_BeginRequest(Object sender, EventArgs e)
        {
        System.Web.Configuration.HttpRuntimeSection runTime = (System.Web.Configuration.HttpRuntimeSection)System.Web.Configuration.WebConfigurationManager.GetSection("system.web/httpRuntime");
        //Approx 100 Kb(for page content) size has been deducted because the maxRequestLength proprty is the page size, not only the file upload size
        int maxRequestLength = (runTime.MaxRequestLength - 100) * 1024;

        //This code is used to check the request length of the page and if the request length is greater than 
        //MaxRequestLength then retrun to the same page with extra query string value action=exception

        HttpContext context = ((HttpApplication)sender).Context;
        if (context.Request.ContentLength > maxRequestLength)
            {
            IServiceProvider provider = (IServiceProvider)context;
            HttpWorkerRequest workerRequest = (HttpWorkerRequest)provider.GetService(typeof(HttpWorkerRequest));

            // Check if body contains data
            if (workerRequest.HasEntityBody())
                {
                // get the total body length
                int requestLength = workerRequest.GetTotalEntityBodyLength();
                // Get the initial bytes loaded
                int initialBytes = 0;
                if (workerRequest.GetPreloadedEntityBody() != null)
                initialBytes = workerRequest.GetPreloadedEntityBody().Length;
                if (!workerRequest.IsEntireEntityBodyIsPreloaded())
                        {
                        byte[] buffer = new byte[512000];
                        // Set the received bytes to initial bytes before start reading
                        int receivedBytes = initialBytes;
                        while (requestLength - receivedBytes >= initialBytes)
                            {
                            // Read another set of bytes
                            initialBytes = workerRequest.ReadEntityBody(buffer, buffer.Length);

                            // Update the received bytes
                            receivedBytes += initialBytes;
                            }
                        initialBytes = workerRequest.ReadEntityBody(buffer, requestLength - receivedBytes);
                        }
                    }
            // Redirect the user to the same page with querystring action=exception. 
            context.Response.Redirect(this.Request.Url.LocalPath + "?action=exception");
            }
        }