You can set a global Media Pool storage space limit for all existing and future users. Go to the settings under Tools & Administration > Media Pool > General. You can set the maximum available storage space in the "Storage space (Limit Default-Value)" section.
Additionally you can set individual storage space limit for each group. To do so, go to the group, click on the blue action button and on "Settings". In the "Portal Master: Settings" section you can set an individual storage space for the current group.