This site http://www.sizeasy.com/page/sizeup is a 3D visualization tool that lets you compare the dimensions of different items.  You can enter in the dimensions of the products you want to compare and also see them compared to some everyday items.

It may not be useful for all types of EDC items (such as knives and flashlights), but it looks to be pretty good for comparing the sizes of PDAs, MP3 players, cameras, and other types of electronic items.

Jeff