1. An image processing apparatus, comprising: a display unit that displays an image of a measurement object and an image of an object which corresponds to the measurement object and has a 3-dimensional (3D) shape which is calculated in advance; a designating unit that designates a first point on the image of the measurement object and a second point on the image of the object based on an instruction input through an input device; a calculating unit that performs a geometric calculation of a first figure based on the first point and a second figure based on the second point; an adjusting unit that adjusts a pose or a position of at least one of the image of the measurement object and the image of the object based on a result of the geometric calculation; and a measuring unit that calculates spatial coordinates on the object corresponding to a measurement position designated based on an instruction input through the input device after the pose or the position is adjusted, and calculates a size of the object based on the calculated spatial coordinates.