The short answer is, according to the most recent research: what they think men find attractive. The research only included heterosexual women, which is why the word is “men,” but can be easily taken to mean prospective partners.
The research by Andrea Meltzer and James McNulty, published in the journal Social Psychology and Personality Science in January, 2015, found that women’s self-perception could be dramatically manipulated by first giving them information implying a specific male preference in regard to weight/clothing size.
If the women were first told that men find ultra-thin women attractive, it had a significant negative effect on their self-perceived attractiveness when compared to the two control groups. The other experimental group, which was told men find average sized women attractive, were more satisfied with their appearance than all the other groups.
Women, like all people, feel most comfortable and attractive when they feel like their environment confirms this. Contrary to popular myth, most men want acceptance, and mutual attraction, love, and emotional stability in their relationships (according to a 2008 study from Iowa University by Christine Whelan).
The most important characteristics in a potential partner were actually pretty much identical for men and women in that study. Few people are looking specifically for thin, and both genders are equally attracted to earning power and ambition according to a 2008 study at Northeastern by Eli Finkel. Whelan confirmed this again in a seperate study.
Advertising is designed to make people feel inadequate to increase a desire to subdue this feeling through purchasing said product, it’s designed to manipulate people into being unsatisfied with what they have and are right now. Distancing yourself from these perspectives, or being aware of its divorce from reality, would likely benefit the self-image of most people. As much as we like to tell ourselves the opposite: what we see has a real effect on us.
The fact is: there is no size or shape that “men” prefer. Men, and women, are simply humans looking for different things. At different times in history, different beauty ideals have emerged. There were periods in history during which obesity was considered attractive, and we’ve been living in one in which thin is portrayed as the absolute ideal.
The truth is: most people are more concerned with real human connection than specific physical attributes. An obsessive concern with specific physical attributes tends to be emotionally unhealthy, so if you are unhappy with how you look: you won’t be missing any really compatible partners.