I have been meaning to mention this site for a few days now.
The site universcale has a flash application that allows you to graphically compare item sizes. At the high end it explains the concept of light years and then using a standard sliding scale the user can go all the way down to a femtometer (which is the scale below picometer and is one quadrillionth of a meter).
As you progress though each scale, images are shown to give you something to compare against. The application also gives you some nuggets of information like: When comparing the same if a nanometer to everyday life, it says that if the Earth was one meter in diameter, a nanometer would be the width of a one yen coin or a little over 3/4 inch.
Even if you have just a passing interest in sizes, this site is a must see. Please go check it out. I'll bet you will be amazed.
Oh, and one more thing. It has a 'new age' soundtrack playing in the background. Luckily you can turn it off...