Size is less important to me than shape, firmness, and attention-loving nipples. That said, a firm, full d-cup can cause me to lose my ability to talk intelligibly. The only breasts I can't get into are droopy ones or overdone man-mades.
They have to fit the girl's frame. I've seen some petite girls get these massive Ds or DDs that don't match the body. For me, Cs or Ds are just right. Bs are fine to if on a tiny spinner.
I tend to like 'em D or bigger, but I also tend to like bigger girls over spinners. As earlier stated, whatever fits your frame. But, I greatly prefer any size natural breast over the fake ta-ta's. Ladies, please keep the boobies real!!!
I put B/C/D because that's my preference, but I must emphasize that this is such a minor issue. Guys have an answer to this like girls have an answer to favorite penile length. However, I bet on both sides of the fence, most (emphasis MOST) do not care so long as it is a reasonable and somewhat proportional size.
Tits. Big ones ,medium ones, little ones, nipples staring right out at ya, like secret searchlights. Mmm. Did I ever tell ya, I like tits!!!!!!!!!!!!!!