It was somewhere outside Barstow when "Chris Carruth"
wrote:

Anyone have suggestions on how thick a diameter the all thread should be?

My bench has ties of 1/2" threaded rod going through it. Like most
bolts in timber they should be large, not because of the strength of
the bolt, but to allow the use of large nuts and washers, so as to
avoid crushing the timber beneath them.

Avoid long metal bolts running cross-grain. You'll get problems with
moisture movement in the timber.

There's little need for long tierods. For clamping a joint together
then a short tie bolt into a barrel nut can be just as effective.

When tieing a mortice and tenon, two bolts are better than one.
Although strength will be good, the rigidity of a single bolt that
relies on clamping forces into a small joint face will be low.

Remember to make the bench dismantleable for transport. Usually rigid
end frames with wedged tenons are bolted onto the long rails. Make the
tenons in the rails as deep as possible and use two bolts. When
planing, this is a joint that takes a lot of racking stress.