Magnetic anisotropy at the interface between a thin metallic ferromagnet and an insulating oxide layer has been used to make devices with voltage-controlled switching [1-4]. Sharp switching thresholds have previously been reported for devices below 100 nm , but questions remain about the size dependence of the switching thresholds and the stability with respect to thermal fluctuations. Here a series of CoFeB (1.5 nm)/MgO (2 nm)/CoFeB (0.8 nm) magnetic tunnel junctions (MTJs) with interface anisotropy were characterized using conductive atomic force microscopy (CAFM) to measure their resistance as a function of the perpendicular magnetic field and bias voltage. With a lithographically patterned MTJ, there are two new features, relative to prior work applying this technique to nanoparticles . First, both magnetic layers have a known crystallo-graphic orientation, which in this case leads to perpendicular magnetic anisotropy and increased squareness in the hysteresis loops. Second, features varying over a wide range of device diameters but the same tunnel barrier thickness have been prepared, enabling the systematic determination of size-dependent properties.