BMI (Body Mass Index) - a measure of body fat based on height and weight.
BMI Categories:
- Underweight = <18.5
- Normal weight = 18.5–24.9
- Overweight = 25–29.9
- Obesity = BMI of 30 or greater
BMI CalculatorThere's quite a percentage of mixed wrestling fans who love their wrestlers to be in the BBW category
(I know Kostya definitely likes 'em on the bigger-the-better side) 
This graph shows comparison results of a male-to-female attraction between a general male population sample and BBW/Fat Admirers, based on girls' Body Mass Index:
[teal = general population vs red = BBW lovers][hs width=500]http://upload.wikimedia.org/wikipedia/commons/thumb/9/95/Fa_attraction.svg/800px-Fa_attraction.svg.png[/hs]
Interesting to see a general male population liking the most girls with BMI of around 18-19, which is considered very slim, bordering on being skinny.
BMI is of course a kind of flawed formula and should be looked on only as a rough guide as it doesn't account for one's fat/muscle ratio. FBB's may be judged as obese if looking only at their BMI.
Still, I find I tend to like a woman to have BMI of around 24-25, which is right on the high end of what's considered normal weight, almost crossing over into being slightly overweight.
However, I seem to place a higher importance on girls' proportions and muscle/fat ratio than on their weight/height ratio alone.
Women with developed muscles softened by a small layer of fat, giving that perfect amazonian-yet-womanly figure. If they have curves, nice shoulders, small waist, wide hips and thicker thighs, even better!

What's your preference?