Alan,
So far all of my nectarines have been small, which has been a bit of a disappointment to me. As I recall, Fruitnut mentioned he could grow some 3"+ nectarines but mine did not approach anything near that big, even with the wet summer we had.
I’ve just harvested a few varieties this year. Carene (nothing special), Silver Gem, Hardired (a little bit tart), Honeyblaze (not to my liking because it’s sub-acid). I have a rootstock that produced some 2.5" nectarines and Fantasia looks like it will produce some 2.5" but all the rest were pretty small. Maybe there are some bigger nects out there.
The biggest thing is trying to get a decent looking nect. People, even at most farmer markets, generally buy the first time based on appearance. Of course one can give out samples (and I do that some) which will sell good tasting fruit, regardless of appearance, but most people are willing to try only big round peaches which are blemish/spot free. They won’t buy donuts at all unless you give them one to try, then they’ll those and many times ask for more the next week. Nects are far from blemish/spot free and are hard to sell. I can see why most growers around here don’t grow nects. I doubt I’ll plant anymore new ones myself.