@germanius: No, you mix that up with traffic quota. Disk quota is available for quite some time and also in 3.0.1.6. @kholy: Just enter the value in MB into the quota field of the website and click save.
Till, I thought he meant quota for a user, not for a website. In 3.0.1.6 it is not possible to set quota for a whole account/user, it will be available with the release of 3.0.2 http://bugtracker.ispconfig.org/index.php?do=details&task_id=851&project=3&tasks=&due=21&status[0]=&pagenum=3 Am I wrong? Sorry if so!