We consider the uniform motion of a drop of one fluid moving in another immiscible, unbounded, fluid. We examine the condition of normal stress balance across the interface. Consistent application of this condition to linear order in the velocity requires the introduction of a new parameter, Δ, which takes into account deformations of the drop from a spherical shape. Imposing a condition for the critical size of these deformations allows us to predict the maximum size of the drop for a given terminal velocity. For the case of raindrops this gives a maximum size of approximately 2 millimeters.