We numerically analyzed the transport characteristics, as a function of the channel length, of MOS transistors based on graphene ribbons randomly doped with substitutional boron atoms. The study was carried out through self-consistent quantum simulations based on a tight-binding atomistic model, validated through comparison with ab-initio DFT calculations. The numerical complexity of the calculations, increasing with the length of ribbons, required the adoption of enhanced convergence schemes. From the results of our simulations, performed for ribbon lengths ranging from 10 nm to 70 nm, we were able to observe a strongly localized transport in the OFF operating region and a diffusive transport in the ON operating region. This made it possible to extrapolate an analytical, nearly exponential relationship between the ION/IOFF ratio and the channel length. In addition, by increasing the length of the channel we observed an enhanced electron–hole asymmetry in the ID(VGS) characteristics and a widening of the voltage range corresponding to the mobility gap. The results of our simulations suggest that graphene-based transistors should strongly benefit—from the point of view of both the ION/IOFF ratio and the device unipolarity—from an increase of the length of the boron-doped channel. Therefore, this solution could be exploited to improve the switching performances of graphene-based devices.