Now this is a tough question... While I maintain that, as mentioned, no boobs are bad, there's definitely a kind of depiction of big boobs that doesn't work for me as much?
In the past, a rule of thumb I would tend to hold to is "not bigger than the head," but that's not a very good rule of thumb at all. Really, for me it tends to come down less to the size of the boobs on their own, but their size proportionate to the rest of the body - at a certain point it can end up feeling more like a body attached to a pair of boobs than vice versa, and that's when I start having a hard time getting into it.