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
