17 inches is probably a bit too short. How tall are you? If you're less than 5 feet, go for around 23-24 inches, 5 feet to 5'6 go for 27-30. Past that if you're really tall, go up to 32 inches, but no more. They're very rough guesses, and I would recommend you to try different lengths until you find the perfect match.
Show us a video of you slinging- when I started my throws made only a 100 feet as well. As you get better, you should be able to get 100 yards. With that short sling, and a good strong throwing style, one might be able to make 100 yards as well, although I think you'd be better off learning with something longer.
17-12 inch slings are really great for tossing paper balls inside however...