If you go too small, you run the risk of having a giant black butterfly-shaped blob on your shoulder.
If you have an artist in mind already, take the picture to him/her. You'd be amazed at how great most of them are and taking the picture you like and modifying it to make a specific tattoo for you.
As soon as my tax money comes in, I'm off to get mine as well! (the boys footprints from birth and their names)