Same here. It looks like the design depicted came from a Benson influence. Here is a look at a traditional english tying joint Clark eludes to.

http://www.tfguild.org/joinery/part2.pdf

6x6, imo, are too small, mostly due to not much room for joinery. 7x's would be about the limit from my perspective. If you have two connectors coming together at the same point on a 6x6 post, it leaves less than a 3 inch tenon and no room for square rule housing. And much less wood around the joint for the strength I would want to see.

Tim