units

Here's a bit of a strange question, though perhaps I'm missing something
obvious..

Let's say that I have a document like this:

 <svg width="140px" height="100px" 
      style="fill:blue;stroke:black;stroke-width:2px">
   <rect x="20px" y="20px" width="100px" height="20px"/>
   <path d="M20,60L120,60L120,80L20,80L20,60z"/>
 </svg>

The two shapes appear identical, including apparent stroke weight. 
Fine.

Now, how do I do this using any other unit besides px?

-mental