Under no circumstances should anybody buy this. I've owned this software for some years now and recently tried to run a scan of my Time Machine backup disc. WhatSize either hung, crashed and at the last attempt caused a kernel panic.
I emailed the Whatsize support people and this is the reply I got:

"as of version 4.7 or so WS will not scan TM volumes for various technical reasons.
for example TM volumes tend to be extremely large and so WS will most likely run out of RAM
that could cause the panic
another thing is that in crawling the disk WS could stumble on a bad block and cause panic."

Disgraceful. I've downloaded 2 free utilities both of which successfully scanned my hard drive and reported the detail I expected.

As others here have pointed out there are free utilities that work. This is the worst response I've ever had from any developer.

What is MacUpDates policy on advertising software that doesn't work on the admission of the software developer?