ASP.NET

public class ResponseSizeHttpModule : IHttpModule
   {
       public int RespSize;

       public void Init(HttpApplication application)
       {
           application.PreSendRequestHeaders += new EventHandler(this.ResponseCheck);
           application.EndRequest += new EventHandler(this.ResponseMessage);
       }

       private void ResponseCheck(Object source, EventArgs e)
       {
           HttpApplication application = (HttpApplication)source;
           HttpContext context = application.Context;
           StreamReader reader = new StreamReader(context.Response.OutputStream);

           reader.ReadToEnd();
           reader.Close();

           byte[] buffer = System.Text.Encoding.UTF8.GetBytes(reader.ToString());

           RespSize = buffer.Length;
       }

       private void ResponseMessage(Object source, EventArgs e)
       {
           HttpApplication application = (HttpApplication)source;
           HttpContext context = application.Context;
           context.Response.Write("Page size: " + RespSize + " bytes.");
       }

       public void Dispose(){}
   }