Lucky you, you came across a big bag of zippers for a quarter at a garage sale. So one day, you go to put a 7" white zipper in a skirt but find you only have a 9" zipper in that stash. It is really, really easy to shorten it!
This, of course, assumes the zipper has nylon teeth that you can cut. And all smart sewists (I think 'sewers' sounds like the place where Pennywise the Clown lives--Stephen King fans get the reference.)
Okay, close the zipper and measure down from the thing you pull. I have no idea what that's called--probably the 'pull.'
If you want a 7" zipper, measure down 7" and make a mark. Then, using your machine or by hand, sew across this mark over and over. This will create a new place for the zipper to stop. Leaving about an inch below this 'stop,' cut off the remainder of the zipper.
You now have exactly the size zipper you need! And you'll probably never pass up a twenty-five cent bag of zippers again.