Moved provisionally to VirusScan Enterprise for faster response.
I do not believe there is an Official or Advertised maximum file size that McAfee will disclose. To do so might provide malware writers an exploitation opportunity. Certainly it would be dependent on the file type Structure. And the Scan64.exe would likely not have a particular limit anyone is likely to exceed. However, for performance reasons, scanning a many terabyte file may not be advised. Depending on the file structure, would not necessarily catch anything more than the proprietary method already in place.
I wrote some thoughts on the VirusScan Command Line Scanner (VCLS) https://community.mcafee.com/thread/70968 I hope you will find useful.