I’m not sure about that (I hadn’t heard it before, but that doesn’t mean it isn’t true), but I don’t like scions which are too short, as there isn’t much place to hold it from and I don’t want to knock the only bud off it with my fingers…
I did experiment with grafting a much larger scion one time (an asian pear which was close to 12" inches long) and got very good growth out of it. But, one of the dangers of using too big a scion is that the growth could be too heavy and cause the union to snap. It also means that there is a lot of leverage on the union, increasing the chances that a bird landing on it (or you brushing against it) will cause it to break.
So normally I like somewhere in between. 2 buds is fine- I’d use 3 if I have extra/unlimited wood.
True- you can always go from a 3 bud scion down to a 2 bud scion, but the other way is harder (and involves interstem grafting 
).
With smaller wood, just do a bark graft. I used to struggle to do tiny cleft grafts and bark grafting them is much easier.
Here’s a series of pics I made last year showing how I bark graft. Last year was the first time that I did it, but I was pretty happy with the results. I wouldn’t replace cleft grafts with bark grafts if I have pencil-thick wood, but for the tiny stuff it is the way to go.