Allow super user to add quota on drop box resources. Default sakai property dropBoxQuota@org.sakaiproject.content.api.ContentHostingService=Q. Each member of the site, has Q Kb of space in drop box. The total space for dropbox in each site will be Q*members of the site Kb. As in resources you could use different values for different types of sites with "dropbox.quota.siteType" property.
Super user can modify this quota for any site or member of a site by editing folder properties in the same way as resources. The resolution order is user dropbox quota, site dropbox quota and finally sakai properties.