List Size Reporting
Updated 7/27/08 - I realized the pictures didn't show up the first time.
Have you ever used the feature Storage space allocation from the site collection administrator's menu?
That is a pretty nice view of all of the document libraries in my site collection and their size. You can also click the drop down for show only and get a couple more choices.
How fun. Finding large lists or documents in the site collection is easy as pie now!
Enabling this feature
As you excitedly clicked into site collection administration you probably found you don't have this as an option.
Why not? These menus only show up when you have site collection quotas enabled. Kind of a bummer. Typically what I will do if I want this reporting is enable an arbitrary site collection quota. This is done from Central Admin > Application Management > Site Collection quotas and locks.
Shane SharePoint Consulting