The file size limit here is set to max file size so that any file size when passed through the firewall that matches the file type should be sent to the wildfire for inspection. If we increase the file size due to each platform having different disk buffer size it may limit wildfire forwarding capacity in terms of number of files. So we either want to send all files with all file sizes to Wildfire(WF) for inspection not leaving any files uninspected or may miss files forwarded to cloud based on disk buffer size. We can always tune this based on our network observation. The Best practice is to set PE file size at 10MB.