Mass Heater Dimensions

We are building a mass heater in the corner of a lean to keep newborn lambs and pigs warm.

Installing the flue on an angle across a corner of the shelter will give us a triangle between 3 and 4 feet on each side, 27 to 64 sq in.

Has anyone had any experience making >wide< mass heaters on a single horizontal duct?
If so how far, each side, of the duct were you able to go. 

Thanks for helping