Quotas are assigned at the time of account creation. It is also possible to increase / decrease quotas after account is setup.
To assign quota:
Login into control panel as host admin.
Click on the Accounts option, present in left frame.
Click on New User button, user registration form will be displayed. Fill the credential of the user, at the bottom of the form enter the quota for this user. Quota value of -1 means that user has unlimited quota while Quota value of 0 means that user has no quota assigned.
Click on the Add User button on form. The new user will be created with the Quota credentials you assigned..