Portable device for contactless measurement of a size, such as the diameter, of small and medium sized objects, such as wires, bars or tubes, even in movement, which comprises a light beam generator (1), two light beam deflector elements (2, 4) located opposite each other, a measuring region (3), an enlarging lens (5), a light beam splitting device (6). The light beam is split into two parts to form two separate images of the object (14) to be measured, being perceived by two linear image sensors (7.1, 7.2) and processed by two electronic circuits (8.1 and 8.2) and by an electronic processing component (9).