It should be possible to set a quota for a site or organization
Portlets like document library, image gallery... etc would only allow you to add files as you don't exceed your quota.
This could be configured in a control panel module, and could be defined by user, role, site, organization or instance
It would be nice to have a graphical representation of the consumed quota for both the admin and the user
This is a development idea proposed by the Liferay Spain User Group (#LSUG) - despite it already existed in JIRA - to be implemented and contributed in case the #LSUG members vote it