If you are asking this question, it is assumed you want to know how to boil your sap faster, and achieve as much productivity as possible from your firewood.
Generally speaking, it is true that a larger fire can generate more heat than a smaller fire. More heat = Faster boil. However, to get the most out of your firewood, we need to give the heat as much opportunity as possible to heat the sap before it escapes through the stack pipe. To do this, long, skinny evaporators will be more fuel efficient than short, stubby evaporators because the heat has to travel a farther distance before leaving the evaporator.
Let’s say you have two evaporators side by side. The only difference between the two evaporators is length. One evaporator is 2′ x 4′ and the other is 2′ x 8′. (Both of these evaporators have the same style of pan, same firebox size, same firewood, etc.) In this scenario, you will find that the exhaust stack pipe temperature will be much higher on the shorter, 2′ x 4′ evaporator. This is because the heat has only half the distance to travel on the 2′ x 4′ evaporator and half as much time to heat the sap before escaping out the chimney. The longer the evaporator, the more opportunity there is to heat the sap and the less heat is wasted.
In conclusion, a big firebox allows you to build a large, intense fire. For greatest fuel efficiency, it is best to pair that intense fire with a longer evaporator.