Research by doctors around the world has indicated that smoking can reduce the size of the penis and the subsequent erection in addition to the other disorders that it causes. One of the researches was conducted by Dr. Pedram Salimpour at the University of Boston. Smoking has been known to damage the blood vessels responsible for carrying blood to the lungs. Similarly smoking affects the blood vessels carrying blood to the penis as well.
It has been found out that the blood vessels such as veins of the penis are much smaller than the blood vessels of the lungs and the heart. As a result, any blockage in these blood vessels may lead to more serious complications. Some people find it more embarrassing to have a smaller penis size and many of these people exclaim that they would rather die than live with a small penis.
Also a very interesting fact brought forward by Clive Bates (who is the director of Action on Smoking and Health) is that it is very hard to imagine that the penis has shrunk whereas the damage to the heart and other vital organs inside the body is much less obvious unless something severe happens. Also the penis is not able to communicate any signs of disorders such as small size which could prove to be a major disadvantage.   
Hence researchers have known for sometime now that cigarette smoking leads to damage of blood vessels which can adversely affect the relative size of your penis. Smoking has proved to be a very important factor affecting the size of the man’s penis. Smoking also leads to smaller erections than before as natural blood flow to the veins around the penis is hampered to a great extent.   
The effect of smoking on the penis can be as severe as its effect on the heart. The blood vessels of the heart are almost 50 percent bigger than those of the penis. Hence we can logically conclude that blood vessels in the penis get affected quickly. The damage to the blood vessels leads to less oxygen supply leading to the formation of collagen. As a result, the elasticity of the penis is severely hampered. Hence when the ideal ratio between collagen and smooth muscle is upset, it can lead to the penis becoming less elastic than before. Also your penis will no longer be able to stretch as smoking has damaged the penis’ response to blood flow.