In this paper we show that silicon split-contact magnetoresistors with geometric dimensions of the order of the Debye length (L_D) can have much higher sensitivities than the usual devices with much larger length and width than L_D. Numerical simulations carried out with Comsol Multiphysics show that silicon n-type magnetoresistors with dimensions of the order of L_D can have magnetic sensitivity as high as 60%/T which is ten times higher than usual sensitivities.