The frequency response and bandwidth of multimode silica glass fibers are investigated in this paper. The theoretical model incorporates both wavelength and modal effects including power coupling from random microbends. The 3-dB bandwidth is examined through the study of the fiber transfer function which introduces the wavelength and modal effects as two separate filter functions. The formal derivation of the chromatic transfer function is analytical. On the other hand, the modal bandwidth is obtained by numerically solving the power flow equation in the frequency domain using the Crank-Nicholson method. As an application, the transfer function is illustrated and subsequently discussed with special focus spent on analyzing the influence of the fiber size in combination with the launching conditions. We show in particular that the larger the core, the greater the bandwidth potential of the fiber when operated under selective mode excitation. Some measurements are also carried out and excellent agreement between this model and data is achieved.