There are no size limitations when uploading a file into Pebble+, unless any have been set by your university or organisation.  All files are compressed and formatted during the upload process.  It is important to note that very large files may take a long time to upload, especially if you are on a slow internet connection. Sometimes the upload process will timeout before it is complete.  If this happens you will need to go through the upload steps again and perhaps move to a better internet connection.