There’s nothing magical about the default value: I just picked a value large enough to hopefully avoid false positives (it means you can’t have the same directory name repeated seven times, which seems even more “giant blob of crap”-esque than my terrible mess.
If you know you don’t have any circular references, you can make it as large as the deepest folder structure that you have.
There is the “library metrics” on the about page, and if you want to get fancy (and you’re comfortable with the terminal), you can use list: https://photostructure.com/server/tools/#path-names-in-your-library:
./photostructure list | grep /path/to/giant/blob/of/gold
will return the images that were imported. To get the list that wasn’t imported, see https://photostructure.com/server/tools/#which-files-in-a-directory-didnt-get-imported