The answer to this question has convinced many women into thinking that they need breast implant surgery or some sort of breast enhancement. The quick answer is that all breast sizes are attractive, because the taste of men is so widely varying. What one man says is attractive, another will say is not. Mainstream media would have the world believe that bigger breasts win in any situation. While it may be true that many men prefer larger breasted women, there comes a point when it's too much. So what is the most attractive breast size.
It varies depending on which era you're in. For example in the Victorian Era women tied their chest up tight with a heavy-duty corset. They wanted to give the appearance of being flat-chested. In modern times you do not have to look far to see what Hollywood and advertising executives think is the most attractive breast size.
Biological reasons have even been cited as to why men prefer larger breasts. They say that on a reproducible level males will choose females based on their perceived ability to feed their children. Larger breasts equal larger depths of breast milk, so that argument would go. And if we use this type of ratione, a survival of the fittest situation would mean that women's breasts are evolving to be bigger and bigger, as larger breasted women would mate more than smaller breasted women. A quick look around reveals that breasts still come in all shapes and sizes, so this does not hold true.
But the real world usually does not have to say according to these biological, evolution-based theories about breast size and men's preference. There are guys that are "breast men" and guys who are "butt men". The latter prefer a woman with nicely shaped buttocks and would choose a nice shape on bottom over size on top. So the saying beauty is in the eye of the beholder exclusively applications in this instance.
When asked what breast size he likes most, Fred Durst from the alternative rock band Limp Bizkit, simply said "Real." In this case, he does not care what size they are, just as long as they have not been tampered with. This would build a case for not getting any kind of work done to your chest unless you have a very dire need. Many women have developed a complex about their breasts, and even though they look in the mirror and see inadequacy, in reality they have a very nice pair.
Searching online for the most attractive breast size is a futile effort. The most attractive breasts are attached to the most confident women, so rock what you have and you'll attract the man who loves what you've got.