The diameter distribution of fibers plays an important role in the performance of many fiberglass products. The fiberglass industry, however, currently lacks the advanced instrumentation necessary for performing on-line fiber diameter measurements. Enhanced measurement capabilities would provide manufacturers and researchers a tool to improve fiberizing methods, production efficiency, and overall fiberglass product performance. This paper describes the size measurement of moving glass fibers using the Phase Doppler Anemometry technique. Simulation results are presented and validated experimentally. Theoretical modeling uses the mathematical solution for a tilted circular cylinder interacting with focused Laser beams. Experimental laboratory studies have been also conducted to evaluate the factors not included in the theoretical models.