I recently handled a case where customer wanted to know all of his available backups and the disk usage of those backups. The script I used in this situation is given below.
Script to list Backup Disk Usage in Plesk
if [ -d /var/lib/psa/dumps/domains ]; then echo -ne "nn=== PLESK BACKUPS ===n"; cd /var/lib/psa/dumps/domains && du -ks * | sort -nr | cut -f2 | xargs du -sh; echo "[Total `du -hs . | cut -f1`]"; fi; === PLESK BACKUPS === 48K example.tk 20K example123.tk 20K example.ca 20K exampletr.ca 20K 123example.com [Total 132K]
Fell free to modify.