There is no maximum file size or length for video uploads. The longer the video, the larger the file size and the longer the upload will take.
For optimum upload speed, test your upload speeds (data vs wifi) and choose the strongest.
If you switch between data and wifi during upload, the upload may fail. When you restart the upload, it should start where it left off unless it was a recording within the app.
Hope this helps and please let us know if you need further support.