Portable device for contactless measurement of a size, such as the diameter, of small and medium sized elongated objects, such as wires, bars or tubes, even in movement, comprising: - a light beam generator; - a first light beam deflector element disposed frontally with respect to the light beam generator, a measuring region disposed in the path of the beam deflected by the first light beam deflector element, and a second light beam deflector element, disposed in an opposite position with respect to the first deflector element, in order to deflect the light beam received from the first light deflector element in an opposite direction to that in which it is emitted by the generator; - an enlarging lens configured to receive the light beam from the second light beam deflector element and to create a real image of the object; - a light beam splitting device located in proximity to one end of the lens configured to split the light beam of the real image into two parts, each part forming a respective image of the object to be measured; characterized in that the portable device also comprises two linear image sensors (7.1, 7.2) provided with sensitive elements (pixels) disposed linearly; wherein the directions of alignment of the sensitive elements on the respective image planes define respective axes (s 1, s 2) different from each other; and wherein the measurement of the quantity of light occurs along said axes (s1, s2), each of said image sensors (7.1, 7.2) being positioned in correspondence of a respective image plane struck by one of the two parts of the light beam split by the light beam splitter where a respective image of the object is formed, said image sensors (7.1, 7.2) being configured to detect the quantity of light on different points of its surface and to convert it into electrical quantities; - a first electronic image acquisition circuit (8.1) connected to the first linear image sensor (7.1) and a second electronic image acquisition circuit (8.2) connected to the second linear image sensor (7.2); each electronic image acquisition circuit (8.1, 8.2) being configured to detect the electric charge accumulated in the different points of the corresponding linear image sensors (7.1, 7.2) and to convert it into a sequence of numerical values; - electronic image processing components configured to determine values (d1, d2) corresponding to respective lengths of darkened regions, along said different axes (s1, s2), on the two sensors (7.1, 7.2) and values (c1, c2) corresponding to the central positions of the darkened regions on the positioning axes (s1, s2) of said sensors (7.1, 7.2), furthermore said electronic image processing components being configured to determine the actual value of the diameter of the object from the values (d1, d2, c1, c2) corresponding to the lengths and to the central positions of the darkened regions. - a light beam generator; - a first light beam deflector element disposed frontally with respect to the light beam generator, a measuring region disposed in the path of the beam deflected by the first light beam deflector element, and a second light beam deflector element, disposed in an opposite position with respect to the first deflector element, in order to deflect the light beam received from the first light deflector element in an opposite direction to that in which it is emitted by the generator; - an enlarging lens configured to receive the light beam from the second light beam deflector element and to create a real image of the object; - a light beam splitting device located in proximity to one end of the lens configured to split the light beam of the real image into two parts, each part forming a respective image of the object to be measured; - a first electronic image acquisition circuit (8.1) connected to the first linear image sensor (7.1) and a second electronic image acquisition circuit (8.2) connected to the second linear image sensor (7.2); each electronic image acquisition circuit (8.1, 8.2) being configured to detect the electric charge accumulated in the different points of the corresponding linear image sensors (7.1, 7.2) and to convert it into a sequence of numerical values; - electronic image processing components configured to determine values (d1, d2) corresponding to respective lengths of darkened regions, along said different axes (s1, s2), on the two sensors (7.1, 7.2) and values (c1, c2) corresponding to the central positions of the darkened regions on the positioning axes (s1, s2) of said sensors (7.1, 7.2), furthermore said electronic image processing components being configured to determine the actual value of the diameter of the object from the values (d1, d2, c1, c2) corresponding to the lengths and to the central positions of the darkened regions.