Yes, I know this is an Office blog. But I thought I would write a Vista snippet from time to time. If there's anything more misunderstood than Office 2007, it's Windows Vista. Much maligned and poorly appreciated, Vista has features that slip under the radar. Yes, I know UAC (User Access Control) which is immediately obvious, but that doesn't count much for appreciation when the average user (who won't run Linux GUIs like Ubuntu, by the way) doesn't accept that limited user accounts are one of the strongest mechanisms to reduce the risk of drive by malware.
Anyway, here's Amit describing how you can catalogue files by file sizes with Vista Windows Explorer
Traditionally, I've been reliant on Werkema's Space Monger freeware.