Largest Silicone Implant That Can Get Inserted Through Nipple?

I believe that you are referring to a peri-areolar( around the circular dark skin surrounding the nipple) incision and not an incision in the nipple.
I prefer the peri-areolar incision because the scar is well hidden at the juncture of darker and lighter skin. The areola size will determine how large an implant can be inserted. In order to insert most silicone gel implants, I usually need at least 5 cms (about 2 inches). I have inserted implants greater than 400cc via such an incision.
If the areola hemi-circumference is much smaller than that , the choice is to have a silicone gel inserted via a sub-mammary incision or if you insist upon the periareolar approach you may want to consider a saline implant. I can insert just about any size saline implant via a 2.0 cm (less than one inch) incision.

The size of a gel implant that can be placed using a periareolar approach really depends upon the size of the areola and the size of the desired implant. An areola diameter of about 4.5 cms can probably accommodate an implant in the 300-350 range, anything larger will have to go through a different approach. An areola diameter that is larger can accommodate a larger implant.