In a Plesk control panel, when you try to increase the mailbox size for any email account, you receive an error below.
QuoteError: 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.
It is due to Mailbox quota size limited in the Resource Manager of the domain. You are trying to set a value more than the threshold set in the Resource Manager. Below is what you can do to increase the threshold.
Login to your Plesk Panel & access the domain. Access
Statistics >> Resource Usage >> Set Mailbox Quota to a desired value. Click on Ok.
Try changing the Mailbox quota size and it should be functional.