Paul’s 8” system in the Fischer Price house IS very nice! I spent two weeks at Wheaton Labs in 2019… both 6” and 8” work well. J-tubes are more efficient and (I think) easier to light than batch boxes.
Below 4-5” systems are MUCH finickier… too much laminar flow/resistance along the edges of the path of gas flow. 10” or larger gets TOO hot…