Typical optical sensors based on D-shape fibers use standard step-index single-mode fibers (SMF) with a circular core. Multi-mode fibers, fibers with elliptical or rectangular cores, and photonic crystal fibers (PCF) are also used to achieve the best possible sensor performance. However, since the presented sensors differ not only in geometry but, most importantly, in the materials and core sizes, it is difficult to compare their properties directly. In this paper, we numerically analyzed the influence of the shape and size of the core on the sensing performance, with all other relevant parameters being equal.