Looking at the Dyson blog I'm pretty sure he's using 1/2" for ribs and deck beams, 3/4" for gunwales/keel. But I could be and often am wrong. The only parts that require actual bending seem to be 1/2".

www.arctickayaks.com would be the link to Zimmerly's page. All he has are lines in pdfs, but once you've built a sof boat, that's all you need. Every time I change computers, I set up a new folder and redownload all of those pdfs, I consider them to be a very valuable resource.
To date, I've built the Hooper Bay from the book and two Nunamiuts from the plans. Then two more from my own design (basically mashing the Nunamiut and Kotzebue Sound designs. I like that particular style for some reason.