We analyze the trajectory of near-Earth asteroid 2009~BD, which is a candidate target of the NASA Asteroid Redirect Mission. The small size of 2009 BD and its Earth-like orbit pose challenges to understanding the dynamical properties of 2009 BD. In particular, nongravitational perturbations, such as solar radiation pressure and the Yarkovsky effect, are essential to match observational data and provide reliable predictions. By using Spitzer Space Telescope IRAC observations and our model for the thermophysical properties and the nongravitational forces acting on 2009 BD we obtain probabilistic derivations of the physical properties of this object. We find two physically possible solutions. The first solution shows 2009 BD as a 2.9 ± 0.3 m diameter rocky body with an extremely high albedo that is covered with regolith-like material, causing it to exhibit a low thermal inertia. The second solution suggests 2009 BD to be a 4 ± 1 m diameter asteroid with albedo 0.45 ± 0.35 that consists of a collection of individual bare rock slabs. We are unable to rule out either solution based on physical reasoning. 2009 BD is the smallest asteroid for which physical properties have been constrained, providing unique information on the physical properties of objects in the size range smaller than 10 m.