Your email accounts can be easily managed with the help of the Email Accounts menu found on your website's Control Panel.
In the Email Accounts menu, you will see the following Create a New Email Account section. In this section, you can create email accounts for your domain.
On the same page, you will also notice the Settings section. Your email accounts can be fully managed here: you can change the disk space quota for the account, change the password, turn on/off the Antivirus scanner to block incoming malware, turn on/off Spam filtering, access the account's Webmail (Round Cube) and delete the account.
Your mailbox size does not affect your hosting account's disk quota, inode and database size limits.