Norwegian Heater

From surface area standpoint:

5 x 8 > 6 x 6.

This assumes the elbows are approximately the same length.

It's the draft regulation that is important...too much and it cools the pipe and wastes fuel, too little and the living space is polluted with fumes and CO....

https://inspectapedia.com/heat/Draft_Regulators.php

A proper heat exchanger is required for optimal efficiency....

..."The energy efficiency of any heating system is measured in its Annual Fuel Utilization Efficiency (AFUE). ... The higher the AFUE, the more efficient the furnace. New furnaces must operate at 78 percent efficiency or better; some high-efficiency natural gas heating systems use 98 percent of their energy input."...

https://www.aga.org/natural-gas/in-your-home/heating/

The flue length is also important...

https://www.installeronline.co.uk/33384-2/

There is also a question of liability here, an improperly installed gas heater, ie: not according to code(and inspected) with permit sign off, would leave one open to liability and possible criminal charges in the event of accident or failure....a fatal mistake....

So 8" is better if I have proper vent, which I do. This system has been in use for 30 years. I use it in my outdoor shop...one whole wall on the south side is open.

Thank you.

Opening of the north wall is recommended for safe operation.

Wind blows hard from the north. . if it rains I get really wet.

I've set it up and its as safe as it has been in years, all new pipe, new vent, excellent drafting, just the right amount of heat as it doesn't get that cold down here. Thank you all.

I was joking.

"... 8" is better if I have proper vent ..."

I don't think that has been established.

What is the total length of 6" vs. 8"?

Velocity will be lower in the larger diameter pipe, at the same firing rate. So you might get longer residence in pipe, without proportional drop in turbulence, giving improved heat transfer. I’m guessing very similar Reynolds numbers.

I doubt this will be a problem for you but you have to be careful to not take too much heat out of passively exhausting flue gasses. You don't want creosote to build-up in the flue liner.

Here is a nice place for chimney draft information.

This can't be determined from the information given. If you have no damper then the 6" pipe will slow down the flow and produce more heat in the stove itself than the 8" pipe. The color of the pipe is important. Black will radiate more heat than the ordinary silver colored stuff. Others have given good information. Finally, if it isn't broke don't fix it.

Neither, stove pipe doesn't produce heat.

But will radiate absorbed heat from the exhaust gases of the fire. I also think that not only surface area but total mass may have some bearing on the matter.

Here is a link for your consideration that may be beneficial in your case.

https://www.motherearthnews.com/diy/wood-stove-catalytic-converter-zmaz83jfzraw

split the pipe and double up the flue pipes

so go with 12 pipes (2 rows of 6) if you have the room.

Thank you all. I've taken your ideas and have the system up and running. Plenty of heat from the 8" piping and no exhaust leaks. I am using propane and can regulate the flame as needed. once again, thank you all.