I was curious to see the dimensions of the iPod Nano in relationship to the Watch. So, I made a little comparison:

It's interesting to note how similar they are in form for being so different in function. It's fun to think that when the Nano became a popular enough "watch" for Apple to give it watchfaces, Apple was in all likelihood working on the Watch already. Based on how similar in size they are, maybe they started with the Nano form factor?