Mailbox size limits within SmarterMail are completely configurable on a domain and user level.
As the System Administrator, you control the domain's disk space size. To configure this limit, log in and edit the desired domain. Click on the Limits tab and adjust the Disk Space. The Domain Administrator then has the ability to allot their users a certain number of space as well, with the System Admin limit taking precedence. To adjust user limits, log in as the Domain Administrator and go to Settings > Domain Settings > Users. Edit the desired user, click on the User tab and adjust the Mailbox Size Limit.
These can be configured in the Domain and User Propagation settings as well to quickly and easily populate these settings among domains and users.