Lateral variations in the Schottky barrier height (SBH) formed at Au/PtSi/(100)Si and Au/(100)GaAs diodes were measured on length scales ranging from a few to several hundred nanometers using ballistic electron emission microscopy (BEEM). All of the contacts investigated showed SBH spatial inhomogeneity. The most severe SBH variations observed were 0.09eV/0.7nm in Au/(100)GaAs contacts and 0.08eV/14nm for Au/PtSi/(100)Si contacts. Based on the lateral maps of the SBH at each interface, the difference between the locally averaged SBH and the globally averaged BEEM SBH was computed. This analysis showed that there is a critical diode length scale below which the SBH deviates significantly from the SBH averaged over a macroscopic length scale. This result implies that the uniformity of electrical characteristics of arrays of small devices (e.g.. PtSi/Si photodetectors and GaAs FET gates) can be expected to deteriorate significantly when device dimensions decrease below the critical length.