I think the answer to this lies in the porn that men look at. All porn is not slim, model-like girls. In fact, the more popular sites are the ones featuring amateur girls; the ones who are the girl next door, the girls that we see every day.
It's all personal preference. So, personally, I like a wide variety of girl figures. Excluding only fat girls. My definition of fat, is girls that are carrying more than 30 to 40 pounds of what they should and have too much of that dimply, cellulite thing going on.
I like boys who are very skinny. My boyfriend tells me constantly that he likes girls that look healthy, otherwise it doesn't matter to him.. That said, I don't like my boys underweight, and my boyfriend doesn't like any body shape that's either too skinny or too heavy to really be healthy.