Decreasing the diameter of a blood vessel by ½ would have a greater effect on vascular resistance. This is because according to the Poiseuille's law, resistance is inversely proportional to the fourth power of the radius (diameter) of the vessel. Therefore, reducing the diameter of a blood vessel by half would increase the resistance by a factor of 16. On the other hand, doubling the length of a blood vessel would have a lesser effect on resistance, as length is only directly proportional to resistance.
