I'd go 1.25" or 1.5". If you decide to use copper, put a union and a valve in so the flow can be stopped when you need to. That will be a huge improvement over a siphon system. On a siphon method it only takes 1 bubble to shut the siphon down, then you need to suck the air out of the high spot before it will resume working.
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.