There are many genetic factors leading to the size of the penis, so it's impossible to actually give a direct proportion for cock size in relation to other body parts. However, here's a few findings:
1. Age actually increases the length of the penis slightly, due to continual stretching via gravity. Don't get your hopes up yet though, because the rate is something along the lines of 1 inch for every 50-60 years.
2. Getting fat decreases the length of the penis, because the mons pubis (the protective layer of fat around the base of your penis) becomes thicker. It varies by body type, but on average losing 40 pounds of fat will make your penis an inch longer. This explains why so few of the old fat guys at the gym look very hung. :tongue:
3. Better cardiovascular health makes the penis larger. People who exercise, don't smoke, don't have high cholesterol, don't do drugs, have regular blood pressure, etc. have larger penises because they have more blood in the penis at any given time (sometime just converts someone from a grower to a shower) which leads to larger hanging and harder erections. Also, some of the activities listed above (smoking and drug use in particular) can shrink the capillaries of the entire body, including the penis, inhibiting general blood flow.
4. Medicines are drugs as well, and some of them can take their toll on the size of the penis. In particular, drugs which lower cholesterol (ironically) can cause penile shrinkage on a temporary basis. This is because it inhibits your body from producing certain hormones and natural steroids in order to decrease cholesterol, and this inhibits the production of testosterone.
5. Although it has been said in "common knowledge" that you can't determine the size of a member from other body parts, the reason why these myths have entered into common lore is because they have some basis in fact. Some years ago, the beginnings of the Human Genome Project discovered and highlighted the Hox gene group, which was determined to be responsiblefor the size of the hands, feet, nose, and genitals. There is a connection between the sizes, but because other factors are involved, people maintain no analysis can be made. Kind of like the multi-million dollar study to determine links between penis size and ethnicity that got swept under the rug for being racist when the white scientists found out they weren't the biggest.*
6. Again, common knowledge has said that there is no way to enlarge the penis. Medical knowledge said the same, then released such products as Enzyte to do just that. Don't believe everything you hear from medical science when they say something is "impossible". "Impossible" is medical jargon for "we haven't figured out a medical and marketable method of doing it yet".
Anyway, sidetracked with FDA rant. The truth is, anthropological reports examining the penis size and sexual habits of other ethnicities are actually quite common. More than a few of them confirm the existence and validity of the Arabic technique known as "jelqing", some reporting penises of 10+ inches as being quite common, and a few anthropologists eagerly encourage their readers to ensure enlargement of their sons' penises.
There you have it, just a little science to toss into the discussion.
*Footnote: So who was the biggest? It was the Ethiopians, which was somewhat ironic. They are black, which fits the stereotype, but their facial structure is distinctly Caucasian, and many Ethiopian skulls look like more European than African stock, just adding more fuel to the racial debate. I think that somewhere, God is laughing at us.