Reducing the length of a kayak by as much as you are suggesting can have a serious effect on the stability. If you reduce the length of a 17' kayak to 14', it will have a lot less stability.

A quick experiment with Kayak Foundry gives an indication of how much effect it might have on one design. Other designs would be affected to a greater or lesser degree.

The 17' default kayak in the program has a SF of 98 and a maximum righting moment with a paddler your weight of about 15. When that design is shortened to 14', if nothing else is changed, the SF goes down to 78 and the maximum righting moment is about 12. That is very tippy for a sea kayak.

Just because the Night Heron at 20" feels safe does not mean that all 20" kayaks would feel safe. The Night Heron was my first strip built kayak, so I know what it feels like on the water. The Night Heron is much more stable than most 20" kayaks, partly because of the cross section of the hull, but mostly because the bow and stern gain quite a bit of width fairly quickly. In other words, the ends are quite full. A 20" kayak with very fine ends would be quite tippy.

You would be much better off starting with a kayak that was designed to be 14' long. Otherwise you might finish up with a kayak that is very difficult to keep upright. It is fairly easy to make a 17' x 20" kayak stable. It is quite difficult to make a 14' x 20" kayak stable.