Í have no official info on this. Most of the Desktop App Configuration is done in this file: C:\Program Files (x86)\DocuWare\Desktop\DocuWare.DesktopService.exe.config.xml - however, I can not find a setting for maximum file size there.
Keep in mind that if you make adjustments in that file, they will be overwritten with the next update for the Desktop Apps. To keep your changes, please put those in a file called "DocuWare.DesktopService.exe.config.overrides.xml" - this will override the default settings from the config file and it is not touched by an Update.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant