Just in case any one is still experiencing this issue, it can now be updated through MySQL using the updated SQL query:
where user@mydomain.com is the email account in question and 1000 is the new size in MB
Code:
use sentora_postfix;
update mailbox set quota = 1000 where username like "User@mydomain.com"where user@mydomain.com is the email account in question and 1000 is the new size in MB
