I dont think you would find an enormous difference in between 10 and 12, at the worst you can keep a steep bow and stern and maximize the waterline that way, As for beam... keep some proportions so it wont look like a point ended pram, if it looks right with the beam you want then its right, your intentions of usage (load) will give you basic dimensions and shapes. Any body of water to me its a real broad term and you might want to restrict it a bit.

you can find some material at:

Sulawesi sailing canoe (outrigger, short but apparently fast, doubt light for how they build them) http://www.mit.edu/people/robot/lepalepa/index.html
(ill spare the roofing tin canoe, even if its pretty cool actually.

still some sailing but "conventional" canoes although give a look at the shape of bow and stern in the photos of the clinker one, its straight up, and then go at the last pic: a 7ft sailing canoe http://homepages.rya-online.net/ocsguk/ocsg_more_photos.htm

And at the end someone that has already built something:
the 10ft "Comanche" http://www.duckworksmagazine.com/06/projects/comanche/index.htm
the 12ft "Baby Raven"
http://www.duckworksbbs.com/plans/sf/canoe/babyraven/index.htm

Another approach could be giving a look at how the guy at capefalconkayaks.com built the SC-1 (a coaster replica) and take that basic shape as start for your project keeping the mods mostly out of the water changing just whats essential under the waterline.

And, well, i guess you have seen the Morris retrieval at paddleandoar.com

: hey all,
: This winter I would like to try making a concept again. I would like to make
: a small craft that is stable, tracks and has enough speed not be
: frustratingly slow. Very easy to store and very lightweight. Something
: that can be put into any body of water. My current thought is something
: between 10 and 12 ft (smaller is better) and open cockpit like a covered
: canoe. Basically a retreval kayak (ala Morris) but in a covered canoe
: form. So my questions: Has anyone tried such a thing (SOF or otherwise)?
: What dimensins would work best? I would love to keep it as short as possible
: but am willing to go up to 12 for more performance. Has anyone built
: something in the 10-12 ft range that can comment on how much of a
: difference there is between 10 and 12 ft?
: The width. I'm thinking between 26 and 29 inches. If Morris can get away with
: 24 inches, is there any reason that 26 or 27 would not work?
: Thanks,
: scott