On Linux, Plesk mailbox is stored in the directory specified in value to parameter PLESK_MAIL NAMES_D in configuration between the file like:
/etc/psa/psa.conf.
By default, the path is:
/var/mail/mail
Names for both Qmail and Postfix can be changed the location of mailboxes.
To limit the size of the Plesk mailbox on Linux:
Plesk bin mail -u [email protected] box_quota 50M