I run a 2X3 (homemade) and my wall comes with 3 inches of the bottom of the back of my pan, the hollow is filled with sand and firebrick and then a sheet steel piece slopes up to my flue, leaving only a couple of inches of clearance for the smoke/heat to escape. With AUF I get about 10 GPH. You definitely want to re-split your bigger logs to wrist size. Bigger burns slower, smaller burns hotter. Its flame you need, not coals.