Here's what Sheldon Brown has to say:
http://www.sheldonbrown.com/biopace.html

I've been meaning to put a Biopace on the front of my Surly. I wanted to move up from the 38x16 that it came with,and my father had an old 42 laying around. I haven't actually done it yet because my chain is to short. Can anyone tell me how to calculate chain length?