Plesk allows us to customize some parameters of our subscriptions directly, without having to associate them with any service plan. To do this, we must enter the subscription we want to customize from the Subscriptions section, and then click on the subscription.

Inside, in the panels located on the right, we have a section that is Account, and we click on Customize

Once in this window, we can manage permissions, mailbox sizes, number of databases for this subscription... For example, to modify the size of the mailboxes, we must go to Mailbox size and indicate the default size of our accounts.