Question: How do I set the disk space limit for a user?
Answer: The short answer is to use the "upload bytes limit" setting.

It's true that the various limits maintained by mod_quotatab, for either bytes or files (or both), are confusing. When designing the module, I anticipated administrators wanting to limit download as well as upload transfers; in reality, most administrators wish to limit the disk space for their users. The bytes uploaded limit doubles as the disk space limit, for there is no effective difference between them; any bytes uploaded via FTP are automatically stored on disk.