Area of a disk 3 inches in diameter : 7.068583 square inches. Times 6 billion : 42,411,498,000sqin. The diameter of an exhaust pipe with such an area is 3.6676 miles. Impressive mental image
It becomes a circle packing problem if you want to visualize 6 billion actual exhaust pipes existing next to each other, and the total area of that one would be about equal to the 3.6676 miles times the inverse of circle packing efficiency (1/0.9069 or 1.102657), so about 4.0441 miles in diameter.
Area increases quadratically with diameter :
Area of a disk 3 inches in diameter : 7.068583 square inches. Times 6 billion : 42,411,498,000sqin. The diameter of an exhaust pipe with such an area is 3.6676 miles. Impressive mental image
It becomes a circle packing problem if you want to visualize 6 billion actual exhaust pipes existing next to each other, and the total area of that one would be about equal to the 3.6676 miles times the inverse of circle packing efficiency (1/0.9069 or 1.102657), so about 4.0441 miles in diameter.