We introduce a simple and computationally efficient procedure for modeling the current flowing through a resonant tunneling diode for devices of small lateral dimension. We include effects due to the electrostatic boundary conditions imposed by the method used to define the device dimension. Our technique combines a commercial device simulator with a physics-based parametric procedure for calculating the current distribution laterally across the resonant tunneling diode. For a specific example in which a 200nm n-type collector is surrounded by p-type regions to confine the current, we find that the peak-to-valley current ratio is degraded from 26 in the absence of two-dimensional effects, to 2.8 when the lateral boundary conditions are included. While our calculation is for a specific method of defining a small tunnel junction, similar effects are expected to occur for most other methods unless special care is taken.