Adjust email space for email account in Smartermail v16
1) Log in to the web interface as administrator through URL: mail.yourdomainname.com with administrator username with password.
Note: The email account need to have administrator privilege in order to adjust email account disk space from Smartermail
2) Go to the last button of the top panel > Domain Settings
3) Select the Accounts on the Left panel
4) Click on the email account from email account list
5) Edit the Mailbox Size Limit (MB) and Save it
You can read more here.