I have an old farm house in Southern Vermont in need of repainting. Should
the clapboard paint finish be flat/eggshell/satin/ or what?? Any advice is
appreciated.

It should be exterior paint, which I thought only came in flat. I
would make it as authentic and historic looking as possible, which I
think would indicate a muted flat tone of color. Which sounds boring,
but really, the bigger point is to paint it in such a way that people
in your area would be more likely to buy it if and when you go to sell
it.