Hi there, there has been some research into this question. Research has in fact found that smoking can reduce the size of your penis.

This finding has come from a study done at Boston University's School of Medicine. Two hundred men submitted their erect organs to the scrutiny of the ruler and the answer came back: if you smoke, you are likely to have a shorter erect penis.

The effects of smoking on the penis are much the same as those on the heart. It damages blood vessels, inhibiting blood flow. Reduced blood flow or poor quality blood flow is the most common cause of erectile dysfunction, of which, smoking increases the risk. The reason for this is that the reduced blood flow may cause the penis chambers to become smaller. The penis chambers all have cell walls around them. When the blood flow is weak or not at optimum levels, the cell walls start to thicken. As they thicken, the chambers are able to hold less and less blood, so they become smaller. Reduced blood flow also effects elastin, the substance that holds the key to what is widely regarded as the key measure of manhood—the ability to have an erection.

It is also worth mentioning that smoking can result in reduced endurance and stamina, cancer, lung, gum and heart disease, tooth loss, snoring, grey skin, wrinkles, halitosis, lowered sperm count and damaged sperm. Hope this answers your question. Have a look at the link below for more info.