6" is usually the size that is used on a 2x3 arch. The stack will not melt, but it will get a nice patina if you get it hot enough. Study the install instructions for some manufactured 2x3's. Someday you might even decide to add AOF. AUF makes it burn hotter but AOF make it burn more efficiently, burning under the pan rather than having wood gasses igniting at the top of the stack when they get more oxygen.
When that time comes maybe you can find a blower on an old oil burner that has been replaced, the blower will be high pressure and thus create the turbulance in the burn above the wood to get higher efficiency and more heat under the pan.
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.