Log onto your Plesk account.
On your Dashboard, click onto 'Mail' on the left-hand side of the webpage.
From here, click onto the email address that you would like to change the mailbox of.
This will take you to the 'General' section of your mail account. You will be able to set a new mailbox size by going down to 'Another size' and ticking the circle beside it. In the rectangular box beside 'Another size' you will be able to input the new size of the mailbox. You can only set this to a value less than the default value (1GB).