We report on VLBI as well as Very Large Array radio observations of the Type I b/c supernova 2009bb. The high radio luminosity of this supernova seems to require relativistic outflow, implying that the early radio emission was "engine-driven," that is, driven by collimated outflow from a compact object, even though no gamma-ray emission was seen. The radio light curve shows a general decline, with a "bump" near t = 52 d, seen most prominently at 5 GHz. The light-curve bump could be either engine-driven or it might represent the turn-on of the normal radio emission from a supernova, driven by interaction with the circumstellar material rather than by the engine. We undertook VLBI observations to resolve SN 2009bb's relativistic outflow. Our observations constrain the angular outer radius at an age of 85 d to be <0.64 mas, corresponding to <4 × 1017 cm and an average apparent expansion speed of <1.74 c. This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum.