This thesis examines the influence of monomodal vs. bimodal aspect ratio on the effective elastic moduli of composites reinforced by aligned continuous cylindrical fibers. The elastic modulus of a porous material with analogous morphology is also analyzed. For the transversally isotropic fiber reinforced composite, out of the five independent elastic constants, the transverse elastic and shear moduli give similar behaviors, increasing their values with decreasing diameter ratio for small volume fractions and reversing this behavior for higher volume fractions. Apart from this, the axial Young's modulus and Poisson's ratio do not show a significant dependence on diameter ratio, giving very similar or identical values for most of the analyzed cases. The axial shear modulus is found to be higher for greater difference between fiber diameters. Finally, the transverse bulk modulus and the transverse Poisson's coefficient show some minor differences only for some volume fractions, appearing to be largely independent of the diameter ratio. When analyzing the porous material, similar results were obtained. The axial elastic modulus and the axial Poisson's coefficient do not depend on the diameter ratio. The transverse elastic and shear moduli show the opposite behavior to the non-porous case. The axial shear modulus and the transverse bulk modulus show some minor dependence on the diameter ratio, decreasing their values with decreasing ratio. Finally, the transverse Poisson's coefficient shows some dependence on the diameter ratio for higher volume fractions, increasing its value as the fiber diameters become more equal to each other.