How to configure resources limits (like disk space, traffic, etc) for individual domain under a subscription?
Resources can be configured for an entire subscription only.
It can be managed via subscription customization at Plesk > Subscriptions > example.com > Customize (under Account section in the right column).
Or via the Service Plan this subscription belongs to at Plesk > Subscriptions > example.com > [Service plan name] (under Account section in the right column). In this case new values will be applied to all subscriptions this Service Plan is assigned to.
It is not possible to change resources for an additional domain individually.
Take part in Plesk improvement by referring to the following link: https://plesk.uservoice.com/forums/184549-feature-suggestions
The top-ranked suggestions are likely to be included in the next versions of Plesk.