hi frds, can u solve this doubt in file uploading

Question

hi frds, can anyone solve this regarding file uploading, that is when my user wants to upload a file, i set a limit upto 20 mb limit in web config, now when user uploading morethan 20 mb it have to show a error message or some validation wants to show, can anyone guide me regarding this, very urgent plz,.

thanks
regards
ram