To calculate the size of a directory in File Manager, perform the following steps:
1 – Logged in to Plesk.
2 – Go to Websites & Domains.
3 – Click File Manager under the domain you want to manage it.
4 – Select checkbox the folder you want to examine, then click More from toolbar.
5 – Click Calculate Size.
The size of all files and folders in that directory combined, is now displayed on the top.