A question came up in the comments regarding the length of the scion.
When I was a young pup, I tried to economize on scion length. I figured that since I only needed one bud to take off and grow, then that was the optimal length. I am smarter now.
It took some counseling from Doctor "Lucky" Pittman to make me see the light.
Sidebar: Lucky came by his nickname honestly. Lucky regularly performed the most invasive procedures imaginable and has never lost a patient on his operating table. That is an incredible record and when Lucky speaks, I listen.
Lucky explained to me that healing tissue knits from both sides. The callous forming on the rootstock needs to knit into callous forming from the scion side. Callous cannot form if there is insufficient carbohydrate reserves in the scion.
We try to pick scionwood from vigorous trees. We try to pick scionwood that was well exposed to sunlight. What Lucky pointed out to me is that a six inch length of scion will have four-times the carbohydrate reserves as a 1-1/2 inch piece.
If it helps, think of the graft as a marriage between the rootstock and the scion. The marriage has a much better chance of success if the bride brings a dowry than if she arrives at the altar destitute.
Given modern materials to minimize the risk of the longer piece drying out, why wouldn't you use longer scions rather than shorter.
As a practical matter, I graft the entire scion length to the rootstock and then trim to length. In most cases, it is easier to grip and make the appropriate cuts to the base of the scion when it is 9" long than when it is shorter.
Why is the untrimmed scion 9" long? Because that is what conveniently fits in a one gallon, Ziplock freezer bag.
After trimming, the scion is generally four or five inches long.
Other miscellaneous notes
Grafting a scion to a horizontal branch will generally result in the selection fruiting sooner than if you graft it to a vertical shoot.
A longer scion that you allow to develop multiple buds will fruit sooner than a scion where you rub-off or pinch the growing points of all but one bud.
The best material for wrapping or covering a scion is parafilm. It is worth your time to get parafilm if you intend to make grafts with exceptionally long scions.
Exceptionally long scion often benefits from a splint. That is, a long stick firmly taped to the branch you are grafting on to and extending beyond the graft. That gives you something to tape the scion to so the graft union isn't flexed and it gives birds a place to land that is NOT your scion.