Implant "size" is more nuanced than a single number. Getting it right means understanding three linked measurements: volume, base width and projection.
Volume (cc)
Implants are measured in cubic centimetres. More cc means more volume, but cc does not translate neatly into a bra cup — that depends on your frame and tissue.
Base width
The width of the implant must fit the width of your breast and chest. Choosing width first prevents implants that sit into the cleavage or under the arm.
Projection
For a given width, the profile sets how far the implant projects. This is how the same volume can look subtle or dramatic.
Putting it together
Surgeons typically start from your measurements, select an appropriate width, then choose volume and profile to match your goals. Sizers and 3D imaging help you visualise the result. See choosing implant size.