Is it possible to calculate total folder size without having to list the subfolders?

Yes, you would need to set the "Limit display level" option on the "Behavior" page. There are 3 related options there:

  1. "Limit recursion level" - this option causes Directory Lister to stop traversing the folders at the given depth level, so total folder size will be smaller than the actual one.
  2. "Limit display level" - all sub-folders will be traversed and total folder size properly calculated, however files below the given depth level will not be shown.
  3. "Group directories" - all sub-folders will be traversed and total folder size properly calculated, however files below the given depth level will be shown in their parent directory.