Oooh, fun topic! I used to be more of a breast guy, so I've encountered several sizes.
I scheduled a session with one of the well-known super-endowed big breast porn stars just to say I had...and yes, there is such a thing as too big. Personally, I'd have to say when they get so large that they interfere with the natural grace of a lady...too big. This lady I met dropped something, and squatted down to pick it up before I could do so. Her ingrained motion was a squat, not bending over like most people would do. It struck me as kind of sad.
While I still enjoy a nice rack, especially one artfully displayed, my tastes have wandered a bit. Some of the ladies I've seen are A's, some D's, etc. Whatever makes them feel sexy is plenty for me. I find I'm becoming more of an abs guy as I get older.