A novel method is presented for the measurement of optical fibers with diameters of 10-120 μm. The method is based on the detection of resonances in the intensity of dye laser light backscattered from a fiber in the far-field when tuning the wavelength. It has been experimentally confirmed that not only a thick (-120 μm) communication fiber produces Fabry--Perot resonances, but a thin (-13 μm) doubly-clad imaging fiber as well. The obtained accuracy is better than ± 0,1 μm. Also, the basic features of the WIRE GAUGE instrument based on laser beam scanning are shortly outlined.