I think nature has more to do with size than nurture.  Take in to consideration the idea that two tall people are more likely to breed tall children.  If short people only mated with short people for a few generations and tall people only mated with tall people, I believe we would begin to see a very distinct height gap.  I also believe it is possible (given enough time) to almost completely eliminate certain traits through "genetic engeneering."  In the case of humans, this is unrealistic, and somewhat Hitler-esque, but hypothetically speaking, not impossible to imagine.

All that said, I do not believe nature is to blame for the world's growing obesity epidemic.  Like other posters mentioned, there is something to be said for behavioral extremes.

I agree with both of the above posters in that both factors will always play a role.  With that being said, I would lean more heavily to nature than nurture.  A man can dream his child will be an basketball star, but there is very little he can do to ensure his son will be seven feet tall.  Proper care and nutrition can help make sure his growth isn't stunted, but there are some things that we can't control.

It is, without question, both.  I believe this because I am proof that it is both.  I am naturally predisposed to get heavy.  My mom first wrote to my grandmother that I was getting fat when I was 5 years old.  Since then, I've always struggled with my weight.  But I am able to keep my weight down through lifestyle choices.  I lost a lot of weight 13 years ago and have generally kept most of it off since.  It's harder for me than for a lot of people (like my wife), but it's possible.

This is a good question. In many ways the whole nature/nurture debate is a false binary. In the end, it is always a combination of both nature and nurture. However, If we had to pick one, I would say that nature is the more important, as long as the nurture situation is not extreme, such as starvation, malnutrition, or other adverse circumstances. There are a few reasons for this. 

First, if we look at things from a ethnic point of view, there is strong correlation between the size of a person and the ethnic stock of a person. This suggest that size is mostly determined by genetics. So, in some parts of Africa, people are extremely tall, and in some parts of South America, people are short. 

Second, within these ethnic groups, some have access to suitable foods and other things that are necessary for life and still there are limitations. This shows that genetics have a powerful role.