Each individual email box has a storage quota of 1GB. There is no pooled limits for an entire domain - each box is allowed 1GB of space, regardless of the number of accounts.
To maximize your mailbox space, ensure that your junkmail and deleted items folders are emptied on a regular basis. All messages in your inbox may be downloaded via POP for archiving.
If you need any help with this at all, please do not hesitate to contact us.