Most 14 foot jons would have a 15 inch transom. I have very rarely seen a 20 on a jon that size, and they are special purpose, and would cost a bit more. Now, a 15 inch transom only means 15 inches in terms of outboards. With trolling motors, it is measured from the bottom of the head, to the top of the motor. in terms of transom mount, I usually put the top of the motor about 5 inches below the bottom of the boat. Then, if you have the handle level with the transom, you need another 5 or so for the clamp. All in all, a 30 inch shaft on a transom mount should do you good.
Now, if you are speaking of a bow mount, it takes a bit more, but then again, it isn't sticking so high off the deck, like the transom one would. A 36 shouldn't cause a problem, unless it is a vee, with a steep rise, then, a 42 would do. Good luck!