A common substitutional dopant for graphene (and, more generally, carbon materials) is boron, an element of the III chemical group (and thus with only one valence electron less than carbon) that has a size comparable to carbon and presents a quite low ionization energy if substituted into the graphene lattice. In particular, in Reference  we have shown that the introduction of boron atoms in substitutional positions breaks the ambipolar behavior of graphene, thus giving rise to asymmetric device trans-characteristics, opens a mobility gap (i.e., an energy region with very low conductance), and increases the ION/IOFF ratio. While these effects become more pronounced when increasing the doping concentration, the values of the ION/IOFF ratio obtained in the devices examined in Reference  are still unsatisfactorily low. Here, starting from those results, we examine the possibility of improving the ION/IOFF ratio by increasing the length of the device channel. The analysis is performed by means of an atomistic simulation code in which the Poisson equation for the device electrostatics and a tight-binding-based non-equilibrium Green’s function (NEGF) transport calculation are self-consistently solved. The substitutional boron doping is mimicked through a proper distribution of fixed charges. To ensure the successful completion of the simulations, we adopt enhanced convergence schemes with variable convergence thresholds and a recursive scanning of the simulation domain. Our results show that the ION/IOFF ratio, the width of the mobility gap, and the asymmetry of the transport characteristics increase when longer ribbons are considered, due to the cumulative effect of the boron atoms distributed along the ribbon. We demonstrate that this improvement of the switching behavior is intimately related to the different transport regime in the ON and OFF states of the device. We conclude that the adoption of longer ribbons in graphene-based transistors should have a positive effect from this point of view, thus representing a further advancement in the direction of functional devices.