Abstract: A device for direct, visual estimation of dimensions of measurement objects that are difficult to reach, such as organs and foreign articles within human and animal bodies. The human eye gives a good size apprehension when comparing a measurement object with a similar thing of known dimension close to that of the measurement object. The comparison can be carried out on an X-ray picture in which a measurement object as well as a comparison body are reproduced close to each other and on the same scale. Means are provided for fixing the comparison body at the exterior surface of the human or animal body. The comparison body is confined by a series of surfaces of revolution with different sizes within an interval matched to the dimension interval of the dimension object.