The theoretical size can be calculated by adding the areas. Since pi is a constant, it can be disregarded and just use the radius squared portion. 4″ = 4 and 3″ = 2.25. Adding two fours 4 + 4 and two threes + 2.25 + 2.25 gives a figure of 12.5. A seven inch flues’s radius squared is 12.25, which is a little undersized when all units are operating at the same time, but under those conditions, the stack temperature will be at a maximum and will give excellent draft.