The capacity of the vessel and the level of the stacking of the biostock directly dictates the size of the pit fire.
To make things plain and simple todays discovery showed me that filling a drum to capacity is not neccesarily the most efficent way of getting more biochar per run. I had chocked a drum almost to capacity with hard wood and left it ontop of possibly the biggest pit fire I have made todate. When I arrived in the morning the fire was but a few coals and when I tried to remove the drum I found it much heavier than usual. I opened it up and found it was 1/2 done. I rebuilt the pit fire and added the drum for another 8 hour burn.
So although I got around about 100% more char this time it took me 2 days to burn it right.
It took me probably 5 times as long to fill the drum as ussual as well.