Is there a general rule of thumb for peg size? For example, do folks generally use 1 inch for 8x8 on up, and 3/4 inch for smaller timbers? I know there is a lot of engineering I'm trying to oversimplify, and I won't hold anyone liable, I promise.

One rule of thumb told to me was that a tenon should be 1/4 the thickness of a timber. So an 8x8 uses a 2" thick tenon, and a 6x6 uses a 1 1/2" tenon.
Then the peg should be 1/2 the tenon thickness. So an 2" thick tenon could use a 1" peg and a 1 1/2" thick tenon could use a 3/4" peg.
Most people I see doing timber framing seem to most always use 3/4" hardwood pegs and if the joint needs more then they install two.
But I have seen some frames with both 1" and 3/4" pegs based again on tenon thickness.
You should never use a peg larger than 1 1/4" unless an engineer told you to. Another rule told to me at a timber framing school.

Good luck with your project.

Jim Rogers

Jim,

I wonder if there is any good explanation for that...

Michal

A lot of it, I would assume, has to do with spacing. There are rules about where the hole should be placed, as in so many hole diameters off the face of the timber where it is in the mortised piece. And so many hole diameters off the end of the tenon, as to not split or weaken either of these pieces.
If a peg was larger than 1 1/4" then these spacing rules may not be able to be complied with, for example.
Basically there is a point where you are weakening the tenon or the mortise by making the peg too big.
Jim Rogers

It really all depends on the size of the timber and the enginering needed. We use pins ranging from 3/4" all the way up to 2". But for a general rule on spacing is 1 1/2x the pins size off the shoulder (i.e. 1" pin 1.5" off shoulder).