I added this to the page:
if($WA_DFP_UploadStatus["WA_UploadResult1"]["statusCode"] == 20 or $WA_DFP_UploadStatus["WA_UploadResult1"]["statusCode"] == 30 or $WA_DFP_UploadStatus["WA_UploadResult1"]["statusCode"] == 0){
echo "The file you are trying to upload is too large. Please compress the file before uploading.";
}
Would this work?