1) Login 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 Left panel 

4) Click on the email account from email account list

5) Edit the Mailbox Size Limit (MB) and Save it