Disk space used by iPhoto/iMovie libraries on OS X

Solution 1:

iPhoto pictures are stored in ~/Pictures/iPhoto Library
iMovie movies are stored in ~/Movies/iMovie Events (iMovie 9) or ~/Movies/iMovie Library (iMovie 10, name may be different as my system runs on German settings)

Select the library and type Cmd+I to open the "Get Info" window which also shows file size (may take a while to calculate).