Around a medium, anything from B to D/E size, anything smaller is limiting, anything larger (at least in real life) usually means ugly in other ways, implants, or as mentioned above, a limited time offer.
In terms of drawn...small to reasonably large, but not those weird extremes that some artists somehow find attractive to draw, so which ever works for the character, though too flat I tend to start finding a bit of a problem, theres nothing wrong with small as opposed to large :p