What is the largest diameter scion?

[CarolinaZone]

For example if I have  3 inch diameter root stock can I graft a 3 in diameter scion to it? Are there any guidelines specific to different species?

[Jaboticaba45]

I am pretty sure that this can be done, just getting a scion that large is one heck of an ordeal. If I had a rootstock that big, I would just veneer graft onto that with a smaller scion, or even graft onto a smaller branch on the rootstock.

[Finca La Isla]

There are several options.  I would probably favor cutting the rootstock off and then grafting onto a new sprout.
Peter

[ScottR]

I would say all depends on hardness of wood on type of tree your grafting.
I've personally grafted white sapote with 1" diameter wood but had to use small wood plan to get cuts nice and flat. Also on that experience I did whip and tongue graft's and wrapped graft's with thick 1" wide poly tape and on one of the graft's I came back to tree next day to see one of the graft's had slipped out of position (out of graft) because of tight wrapping of graft. Luckily I was able to un-wrap graft and wrap again but not as tight. That's one thing that folks don't think about with plastic tape used to wrap graft's because it has memory (tape). After you stretch it on graft it relaxes back to original size meaning it put's great holding power around graft and can move a whip or whip & tongue graft.