I have built a barrel type oven that is 33" x 34" inside cooking surface.  Barrel shape is 16" high.  I will step down to opening height of 10" x 18".    Will a 6 round flue be adequate?   I was told that I may need 8", but it seems too big to me and seems it will let too much heat out.   Of course I don't want smoke backing up either.  

Thx