The "verbal" upload limit does not matter at all. See Ken's example of the 93 GB file! The point is, there must be technical limits and the infrastructure must be able to handle them. If the limit is 800 MB and the Moodle + Turnitin architecture is such that the whole file is loaded to RAM before sending to Turnitin service, that would be an invitation for a DOS attack!
The limit must be much lower. Turnitin is only a service, as opposed to software which you can run, right? Then there is no chance that they allow files of any arbitrary size. Wasn't there a upper limit in your contract with Turnitin? Is it possible that their plug-in does not respect that limit?
Whatever, this is a matter for the Turnitin people.