This paper deals with the on-line sizing of small diameter glass fibers (i.e. d < 30 μm) produced for textiles and reinforcement applications. Two models based on the Lorenz-Mie Theory are introduced to predict the basic light scattering properties and the response of a phase Doppler interferometer (PDI) to the sizing of infinite glass fibers. Among other parameters, these models take into account particular effects such as the fiber's single-axis birefringence and the fiber's refractive index dependence on its cooling rate (i.e. diameter). Both effects have a weak influence on the mean response of the PDI but a strong influence on the resonance structures of its phase-diameter relationship. Two optical set-ups were selected from a numerical optimization procedure and tested experimentally. Experimental results are presented demonstrating the validity of the models and the ability of the developed PDI set-ups to study some features of the fiber drawing-process: fluctuations of the fiber diameter when the nozzle is submitted to a convective perturbation and, when the fiber take-up velocity is modulated, the detection of hollow fibers.