I want exactly this. It's time consuming and just crazy to have to check each game/app for it's file size individually. It takes ages to do this as well. If Sony are shafting us with these small memory cards (I've got a full 16GB) then they at least need to allow us to manage them easily. DaisyDisk on the Mac is my ideal client.
Speaking of which, has anyone found a nice tidy screen detailing the apps installed complete with file sizes? I thought Settings - App Data Management would be good but alas no file sizes. I just went with the rather cumbersome Information menu option for each app installed on the main menus.