The firebox dimensions are 16x16 and 16x20 respectively. The circulators are a much better stove than the Hitzer 82. But, it's not a fair comparison, the original Hitzers were not designed for people like ourselves, they were meant as simple sources of heat for the Amish community. For example, my model 75 uses threaded bolts bent into a U shape to hold the door closed. The circulators made by Amos and others at DS are much more sophisticated and well-made than the model 55/75/82's that you have been asking about. To be honest, the model 75 that I have was more work than my paying job! I will never sell my Hitzer, but I don't look forward to lighting it, and burning coal should be a little fun after all right?