1. An image processing apparatus for obtaining a scale of a region of a processing target included in an image, comprising: at least one processor; and at least one memory storing a program including instructions executed by the at least one processor to: obtain an intensity distribution attribute of the region of the processing target; select, based on the intensity distribution attribute, a calculation method for calculating the scale of the region; and obtain the scale of the region by the selected calculation method, wherein the intensity distribution attribute defines whether the region of the processing target is a convex region which has a higher intensity value than its surrounding region, a concave region which has a lower intensity value than its surrounding region, or a mixed region including the convex region and the concave region, and wherein, if the intensity distribution attribute defines the region as the mixed region, a component of the convex region and a component of the concave region in the mixed region are obtained, and the calculation method for calculating the scale of the region for the mixed region is selected based on a comparison between the obtained component of the convex region and the obtained component of the concave region.