I am trying this year with a couple of seedlings just less than a year old that are less than pencil thickness (I have other seedlings growing that I am saving until they get bigger before grating on). They are thicker than the scion I grafted on them at the base of the plant though and that was good enough for me.
I don't know if it the correct size but I will find out soon enough
How old and what is the diameter of root stock? Do you have your scions yet and how do they compare?