What sized motor could spin ~2lbs?
Everything really depends on how the sign is mounted to the motor. If the sign is centered, then barring a requirement for top speed or how quickly it will spin up, just about any motor will do. If the sign is not centered then at torque calculation is needed. To calculate the torque multiply the weight of the object by the distance away from the center of the shaft. For example one pound two feet away would be two foot-pounds of torque.