You can set all quota's for a domain with a default setting:
quota_default "500mb"
Then individual quota's can be set in the user settings.
In rare instances you may wish to adjust the quota for all users in a domain, that can be done with this command:
tellmail users_setfield "*@domain.name" quota "600mb"
In the user admin when you lookup a user there is a button to show current disk usage per folder, and to recalculate the totals.