Abstract: The apparatus comprises: a holder for a test piece; at least two optical systems, identifying two optical paths located at a predetermined and known reciprocal distance, which are able to focalize, with a predetermined degree of magnification, images of two ends of the test piece; the at least two optical systems being aligned with the holder; at least one viewing and measuring device able to collect the images which are focalized by the at least two optical systems. The apparatus is structured to perform measurement of a size of a test piece while completely eliminating any influence on such measurement on the part of the holder or the measuring system.