In the restore screen,  as well in Backup as in Archive, the folders are shown without size.

The reason for that is a technical one:
The size of the content of a folder is not constant, it changes for instance dependeing on the index options.

A folder or a subtree thereof may be included or excluded by selecting a specific pool or tapeset. So the folder size cannot be calculated in advance, it would have to be calculated when viewing a folder.

When starting the restore for one folder, that calculation is performed, it is then visible how long that calculation takes. Doing that online would just take too much time, as a result it would be rather time consuming to open a folder in the restore section. In worst case, if there are many subfolders, it may even be possible that the browser times out in the meanwhile and closes the connection.