Maximum File Size

You are correct: there's no hard limit for the size of the files that can be stored in an M-Files vault, but different M-Files functions have their own limits. In addition to preview, at least M-Files Aino, full-text search indexing and PDF conversions come to mind.

Some of the default limits can be raised via vault configuration, see for instance Specifying PDF Conversion Limitations for Indexing and File Preview in the user guide.

See also the support article Is There a Maximum Document Size for M-Files from the Performance Point of View?