To change the mailbox size of an email account, we must log in to Plesk and go to Domains > click on the domain of the email account > enter the Mail or Email Addresses tab:
Click on the email account that we want to change the mailbox size for, in the example ph@profesionalph.com:
Finally, we go to the Mailbox option and select the size we want to give to the email account and click OK:
And we have now changed the mailbox size. If there is an error, first we have to modify the Mailbox size from the Subscription, you can follow this guide