I scrounged up a copy of the plans I used several hundred years ago when I built the first one. They were published in a book of boat plans. They really aren't good plans because the technology has changed enough that there's better ways to build one now.
I started with the rough dimensions from those plans and then went over the edge. I completely redesigned the whole thing, including altering the structure, to come up with something from my imagination. Then I let the wood bits decide where they wanted to go, so it's a collaboration between my imagination and some western red cedar.
I shortened up about a foot from my first mockup and that pushed the stringers out so that it's actually a mite over 26" wide, and very flat floored under the cockpit area. Keeping my fingers crossed that I've made enough good choices that the boat will be useful at least. The last one was a terrific boat, now I'm trying to guild the sows ear...
here's a page that shows the general schematic structural idea I'm working on:
http://yostwerks.com/WoodSOFFrameMenu.html
clicking through this guy's site will get you started, and some more searching will produce a ton of great plans and ideas.
here's another. I don't care for this guy, he's a commie whiney snot nose kind of guy, as near as I can tell from his site, but he has designed a beautiful kayak called F1, and published the plans. It's a different style of construction, much closer to the original Eskimo engineering.
http://www.capefalconkayak.com/f1.html
somewhere on this site are the plans for this kayak. I gotta hand it to the guy, he makes beautiful boats. click around and you'll get the details somewhere.
let me know if there's anything else I can help with.