There's a guy in CA who built the stock design and also did one at the same length but narrowed it down to 19 1/2 wide. He's a racer and said he felt no difference between the two. There's a woman now building a shortened and narrow one but not complete as of yet. (Doing as Brian suggested) You could also bring down the length a bit and keep the forms the same. These tiny tweaks won't affect the speed that noticeably. The boat is quite stable for 21" and bringing it down to 20 won't show up much if at all. Going to 19, you starting to push the edge, but any paddler with average skills would do fine in it. I'm going to build a 19 wide one myself and just a little shorter and lower the volume more as well. but that won't probably be until next winter.

I have women who have built the stock boat and are smaller than you and are very happy with it. Most of the request I get are people with larger feet and who are tall. If you want a very snug boat, don't be afraid to experiment a bit. You won't rune it. That's how I did it. I wish I could give you more exact data, but in actuality nobody could, because paddlers are all different with their feelings about stability , turning, tracking etc. There's no agreement about what's good. It''s what you like.