Star-San, which is the best, IMHO, requires liquid contact. That does not mean that the fermenter has to completely filled. It just means that liquid must be in contact with the surface for 30 seconds. I use about 1 gallon and gently swirl it around to coat the entire interior surface.
FWIW, there is a huge difference between sanitized and sterilized. Short of obtaining an autoclave big enough to fit all of your gear into, you will never achieve sterile conditions.
Sanitizing with a sanitizer (Star-San, Iodophor etc..) will sufficiently reduce the microbial levels enough for our purposes.