Learn to bell curve.
Statistically, the most attractive people are the ones within one standard deviation, σ, on the bell curve. If you graph the bell curve of the range dangerously skinny to dangerously fat, you'll find most people fall somewhere in the middle--and that's where you should aim for.
You also have to understand that each person is different, and even though the popular thing is to be skinny, there are some people who go for fat girls and there are some who go for skinny girls. Again we could plot a bell curve ranging from dangerously skinny to dangerously fat, against attractiveness. Statistically, somewhere in the middle is most attractive, but the nice thing about the bell curve is that there are always some outliers.
What I'm saying is that statistically on aggregate, you're better off somewhere inbetween dangerously skinny and dangerously fat, but I'm also saying it doesn't really matter.
For more about the "bell curve" see: