One of the teaser-trailers for the Sekiro: Shadows Die Twice shows one of the bosses - Chained Ogre. In an interview for Game Informer, Hidetaka Miyazaki revealed that this is his favorite boss in the game.
Although Sekiro developer decided to give up on the idea of battling extremely large bosses, Chained Ogre turns out to be a rather big opponent. In the trailer you can see that his fists are as large as that of the main protagonist's head.
This doesn't mean that all bosses in the game are as big as this one, but it's just a fun observation made by the fans of the game.