pipe diameter for 2x4

[canaanmaple]

picked up a smokey lake 2x4 that came with single wall 7" flue pipe. I want to put double wall through the roof. Stack would be about 12'. Would putting a reducer to accommodate 6" pipe at the top of stack have much impact on boiling rate? 6" double wall is easier and cheaper to find that 7" double wall.....
thanks!

[maple flats]

By double wall I hope you mean a class A insulated chimney. Just double wall in uninsulated will get too hot and could set the roof on fire. On a 2x4 you could likely "get away" with a 6" as long as the stack total is 12', but if you go to an 8" you would then be set for your next larger evaporator, a 2x6 and could use the same thru the roof stack. It all depends on what your plans are for 5 years out.
If you are likely to get a 2x6 in 3-5 years, the long term will cost more now but less in the long run. If you are unlikely to expand to where you need a 2x6 until 6 years or beyond a 6" is likely the best call.