I am in the process of building a forge out of 12” pipe. Gonna be approximately 16” long. Would anyone have a suggestion on what length to make the ribbon burner?

The interior of my monolithic forge is 18" L x 9" W x 10" H, approx. volume of 1600in3 and the appropriate sized burner was the Pine Ridge LP290 with a face of 4.25"x8.25" (perhaps a bit large but it can be dialed down).

Will the interior of your 12" pipe be lined with 2" of inswool? Assuming so, that would reduce your interior diameter to 8" x 16" L, approx. volume of 800in3.   Perhaps the LP190 at 4.25" x 6.25" is the proper fit?

A key installation tip: Total volume is not the most critical design paramater. Instead, make sure the burner face has a minimum of 9" clearance to the flame face. My LP290 has a clearance of 10". This will ensure complete combustion within your chamber, reducing the confusion or need to perfectly match chamber volume to burner size. When you mount the burner make sure to recess the burner back if necessary to give the 9-10" clearance.

Install blower speed controls and gas needle valves to allow dialing in the flow of air and gas.