Norm,

In my experience you are better off relating to peg "hole", rather than peg, diameter. There are a number of reasons for this that are particularly obvious if you are using traditional trunnels.

Pegs (trunnels) are traditionally octagonal (roughly) which means you have to decide whether to measure across the points (looking at the cross section) or the flats. They ar also very often tapered, so you would need to decide where along the length to measure them.

Using a peg that is a little larger than the hole across the points and a little smaller across the flats about halway up the peb is optimum. If the pegs are sawn rather than "riven" (riven is best) then that sizing goes for the entire length beyond the tapered point. This avoids peg "binding" which can easily split a tenon or a mortice cheek. It also allows the peg point to "feed" through the drawbore.

The drawbore is also very important. Drawbore is placing the hole in the tenon a little closer to the shoulder than the hole in the mortice cheek. Depending on how much "relish" (the wood left beyond the peg hole) you have allowed, the draw should be no less than 1/16" and no more than 1/8". Peter McCurdy will also be quick to warn "It's best to avoid having pith in your relish."

Enjoy your timber frame project!

Rudy