I have no experience with webinars.
189GB
I wonder if the file was 'finalised' - can you play it? If I get a PC crash while screen-capturing with Debut, the recording that was ongoing at the time is not playable.
NCH make Prism Video Converter (you may have it in the NCH folder ready for installation) which they say can compress video - you can probably use it on trial.
I've never tangled with such a huge video, but nowadays I record in mp4, for ease of handling, and don't notice a very significant reduction in quality. It would still have been a huge file, just not quite as huge.
A tip for the future - in Options>Recording, you can set Debut to stop and resume recording periodically, so that you never end up with an unmanageable file. You miss a couple of seconds while Debut finalises one segment and starts the next, so you may not want to set the maximum recording time for less than an hour.
http://i2.photobucket.com/albums/y40/satissa/techie%20misc/2014-07-2720_24_07-Options_zpsa8807c13.jpg
Also, the higher your quality settings the more 'heavy' the resulting video will be to handle.
I found someone saying that they use a Firefox extension "Download them all" for saving webinars. If that link doesn't work you can search for it in Firefox's extension page.
I can't understand why a 5-hour video should be that size, though. After all, an HD movie would only be 2-4GB.