What are the smallest & biggest size of Sientra round butt implants available?

Sientra only makes/has FDA approved breast implants not buttock implants. Hopefully no plastic surgeon is proposing using Sientra “buttock implants” on you.?
Unlike breast implants, silicone gel buttock implants are not FDA approved and thus not legal for buttock implantation in the USA. I actually see about one patient a week that had gel implants done in another country and, although they looked and felt great at first, they are now seeking revision because the implants are either visible on top of the muscle and/or grossly deformed by flipping over, rotating, scar contracture and/or the implant has ruptured several years after placement leaving behind hardened scar tissue my migration of silicone liquid. The soft semi-solid silicone rubber implants used in the USA are much more form-stable and thus much less likely to have these problems if placed correctly within the gluteus muscle AND are considered permanent implants because they cannot rupture and leak like silicone gel implants. In 2015 I created and recently patented a new "Stanton Anatomic" shaped implant that avoids the unnatural effects of round implants, whether silicone gel or rubber . The benefit of this is the fact that ~95% of patients have much more of an oval shaped buttock cheek not round, thus a true oval implant (i.e. Stanton Anatomic implant) creates a much more natural and harmonious buttock contour. This is especially true when the patients bends over, does squats, or pokes their butt out while striking a pose or dancing. Many avid weightlifters, fitness competitors, and athletes from various sports have had these implants without problem of movement, visibility, or drooping.

Please note that an extremely few number of board certified plastic surgeons specialize in buttock or hip implants, so you may need to travel far and away from home to find this particular surgeon. Glad to be of help.