The diameter of the artery in normal conditions (in other words variance in size of the diameter) does not make much difference. If there is a varicance in rbc size that is significant this can lead to a rolo (sp?) condition. Also, large rbcs can be a sign of anemia.
If the arterial diameter is decrease due to a pathologic condition (like plaque build up) this can increase the pressure on the vessel to the power of 4 or greater. When this happens, it can have many significant adverse rxns.
this would be the short answer.