A search through a large number of subdirectories is problematic in pure Relevance because the search can take a long time and hang the client from processing anything else until it’s finished.
I’d lean toward an Action that uses shell commands or powershell script to search the disk and save the results to an output file, and an Analysis to parse the results of the output file.
The example at Outlook PST Size looks good to me, have you tried this method, and if so, where did you have trouble?