Blakiston's fish owl is the largest living species of owl. A pair field study of the species showed males weighing from , while the female, at up to , is about 25% larger. Around February, the average weight of Russian fish owls was in seven males and in five females, typically when their body mass at its lowest throughout the year. Blakiston's fish owl measures in total length, and thus measures slightly less at average and maximum length than the great gray owl ("Strix nebulosa"), a species which has a significantly lower body mass. The Eurasian eagle-owl ("B. bubo") is sometimes considered the largest overall living owl species. The three largest races of eagle-owl, all found in Siberia and the Russian Far East, are close in size to the Blakiston's fish owl. According to Heimo Mikkola, the very largest specimen of eagle-owl was longer in bill-to-tail length than the longest Blakiston's fish owl, while the top weight of the two species is exactly the same. The longest great gray owl was longer than the biggest Blakiston's fish owl but would be about 2.5 times lighter than the weight of the largest female Blakiston's. However, the average measurements of Blakiston's fish owl surpass the average measurements of the Eurasian eagle-owl in at least two of the major categories: weight and wingspan, making Blakiston's the overall largest species of owl. Even the large Siberian races of eagle-owl are slightly smaller on average than the Blakiston's, at least in terms of body mass and wing size. In the largest known body mass cited for a Eurasian eagle-owl population, the weights cited, for instance, were a mean for the males and a mean of . The maximum wingspan of the Blakiston's fish owl is also greater than any known eagle-owl. The wingspan range known for Blakiston's fish owls is . It is possible the largest specimens can attain a wingspan of approximately . The Blakiston's is noticeably larger than the other three extant species of fish owl.