The changeable hawk-eagle is a largish but slender eagle. They fall near the middle of sizes among the currently accepted species in the genus "Nisaetus". As in most birds of prey, females are larger than males with an average overall size difference of 7% but this can individually range to an 18-22% difference, with island races apparently thought to be less dimorphic on average. Size is quite variable and total length has been reported in the past to vary from and wingspan from , however these figures appear to include the much more massive hawk-eagles from Flores that are currently considered their own separate species by modern authorities. Nonetheless, total lengths of up to have been listed for "N. c. limnaeetus" in Nepal. Ali & Ripley (1978) estimated these average total lengths for the following subspecies: "N. c. cirrhatus" at , "N. c. limnaeetus" at and "N. c. andamanensis" at . Legge (1880) measured the length of Sri Lankan changeable hawk-eagles ("N. c. ceylanensis") without including the beak as . The average length of birds from the Philippines ("N. c. limnaeetus") was measured as in males and in females. Weights in this species have been reported from but the source of this is unclear and it probably underrepresents the size variation known to occur in the species. The only precise body masses known for the species are derived from the Philippine population, where males were found to average while females averaged but they could weigh in excess of .