A turbofan engine having one or more heat exchangers tied to a power gearbox is provided. The power gearbox mechanically couples a low pressure spool and a fan of the turbofan engine. The one or more heat exchangers have a heat exchanger capacity defined by a resultant heat transfer surface area density associated with the one or more heat exchangers multiplied by a heat conductance factor that relates a power gearbox heat load, a fan power consumption function, a fan diameter of the fan, and a bypass ratio of the turbofan engine. The heat exchanger capacity is between 0.77 and 298.17 for a rotational speed of the low pressure spool between 2,000 and 16,000 revolutions per minute at one hundred percent capacity and a resultant heat transfer surface area density being between 3,000 m2/m3 and 13,000 m2/m3.