Below is a photo of a 475cc Mentor sizer, with the attached fill port. Sizers are disposable implants that some surgeons use during surgery in order to get the most accurate size for the patient. The sizer is placed in the patient, and filled with saline until the right "look" is achieved. The surgeon then removes the sizer, and replaces it with the permanent breast implant that is the same cc amount as the size that was achieved using the sizer.
Sizers are also referred to as disposable breast implants, disposable sizers, temporary breast implants, and temporary sizers.
To read more on temporary sizers, visit the New York Times article covering the option and hear what different surgeons think of the procedure.