I would not use GrandPerspective to locate a file I was looking for in advance; there are better tools for that. But if you are looking for large "things" in general, they can be selected/highlighted even within groupings. My comment "to find the files I want to delete…" may have been misleading. The files I want to delete are probably the large ones GP comes up with, not something I had in mind beforehand.
Thanks for the clarification.
So, you use GP the same way I do...to see if there are any new extraneous items on your drive that are large enough to stand out.
As you've said, you can find anything you're looking for with enough effort, but considering the better tools that are available, it's totally wasted effort.
