I doubt any of you need this but I figured I'd post it anyways.
For a 96 zetec with out a/c and with an esslinger/ford racing UDP, the proper belt size is 6pk1695 or 6pk1700. 6pk1700 would probably work good and be easier to fit. I had 6pk1710, but it was a bit too long, making the tensioner extend to just about its furthest point. It started slipping in the rain fairly soon after I put it on and continued to get worse and slip when dry. The 6pk1695 just fits on, I don't mean you have to force it on, but the tensioner only has to move a little bit to make it tight. I think maybe 6pk1700 would be best all around.