I believe that as long as the bust is proportional to the woman's waist, the girl's body will look fine.
Ideally this ratio should fall between 1.4 and 1.5 (bust/waist ratio).
The cup size determines the 3D-ness of the boobs. This is personal preference but I believe most men will like C, D, or DD cups.
The bust size and the cup size determine the total volume of the breast. If the volume is too large (>36 bust size with a D or DD cup size) then the breast starts to sag (although this is not the only reason why breasts sag).
Going beyond the DD cup means that the breasts will definitely sag because of physics and most people don't like saggy boobs.
However, on the opposite end of the spectrum, some men may prefer smaller boobs if the woman has a small waist because it falls within the ideal bust to waist ratio.
Example 1: Octavia Red (36D cup size, waist 24, ratio=1.5) perfect ratio
Example 2: Anissa Kate (36E or 34DD cup size, waist 25, ratio=1.45), the boobs just start to sag but they still look nice because she's within the ideal bust to waist ratio
Example 3: Dani Blu (32A cup size, waist 22, ratio=1.45) her small boobs fit her small body because she's within the ideal ratio.
