Can I change my 5mm chin implant to a small, should I get a 3mm or 2mm?

 Hi, I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  There is a slight amount of swelling associated with placing a chin implant through an incision under the chin.  You should give yourself 1 month for this to resolve thereby showing more of what the final result will be.   When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  
 Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  
 I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.   I prefer using a size small in women and a size medium EAC in men.  If after the month, you still want the implant to be exchanged for a size small, this shouldn't be an issue.  Much has and is still being written about the scar forming capsule around chin and cheek implants.  This scar is very thin and doesn't have sufficient volume to add any perceptible amount of projection.  Similarly sagging would not be expected to occur when exchanging a size medium for a size small EAC silastic chin implant.
Hope this helps.