To assign a size to the email accounts of a subscription, we must log in to Plesk and go to Subscriptions > click on the subscription that we want to modify > go to the customize option:

We look for the option that says "Mailbox size" and select the space we want to give it, unlimited or the one we customize and click on accept:

In the example, we have set it to unlimited, we can see it by entering the email / email addresses tab of our domain and clicking on update:

We update the usage and see the selected size: