Once web development and design is done, and website is about to launch there is some tricks in Plesk control panel that you might need to know.
Here is the problem: Once you are in selected domain and you go to Mail -> Mail Accounts -> Account -> Mailbox and try to change size of the mailbox quota, following error occurs: Error: Unable to set the mailbox size limit for the domain: The size of this mailbox must not exceed the limit on amount of disk space allocated for mailboxes in this domain.
Here is the steps on avoiding this error:
1. Login to Plesk
2. Select Domain
3. Statistics -> Resource Usage -> Set Mailbox quota to desired and then you will be able to change mailbox quota up to this limit. Once this is done all default mail boxes sizes will be changed to the one you entered.