Old wisdom suggests the open cupola area equals the area of the evaporator. However hoods can reduce that.
In my case, I had a 2x6 evaporator when I built my sugarhouse. I later got my 3x8. My cupola is just 2x6'. My 3x8 has a tight hood on the 3x5 flue pan and a raised hood over the syrup pan, the cupola is plenty big. My sidewalls are 10' tall and my roof is an 8/12 pitch, I don't know how much that might affect the performance. At any rate, my 24 s.f evaporator is happy with a 12 s.f. cupola area. In fact, it's still enough even when I also have my 2x6 finisher running too.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.