The default email storage quota for your domain is 2000MB, but please note that the default Mailbox size per user is only 50MB, this is to avoid unused mailboxes filling up with spam. You may however distribute the 2000MB quota however you wish among your users, but the more users you have, the less space each can have.
PLEASE NOTE: that ones a users quota is used up, he will not be able to receive any more mail. Once your doamin quota is used up. nobody will be able to receive email.
I would recommend to increase this for each user you create to at least 100MB. If you are importing a lot of mail from another mailbox at your old host, then increase mailbox size sufficiently to hold all the imported mail plus new mail.
To change mailbox size This can be done on a per-domain basis, once you have created the domain in WCP, you can log in to SmarterMail as the MailAdmin account then go to Settings>Advanced Settings>Default User Settings. From there you can set the Default Mailbox Size Limit. You may then propagate these changes to your exisitng users if required or edit users individually to give them more or less space.
You may designate any user as the domain admin via WCP, simply by editing that user and setting him as the domain admin.