Largest Silicone Implant That Can Get Inserted Through Nipple?

It depends upon the size of your areola, the thickness and firmness of your tissue, whether or not the implant is textured, and the degree to which your surgeon will force adn distort your implants through the incision.
But this all begs the following quesiton: why? Why would you use that incision?
As I see it, that incision has only one possible benefit: that the scar might be good. And that's it. And it's not a guarantee.
But what if the scar is bad? You'll see it everytime you look straight at your breasts. But the underneath incision is hidden by your breast, and is generally visible only when looking from under your breasts when your arms are over your head.
Beyond the scar, the advantages of the underneath incision relative to the areolar incision are overwhelming. One recent study showed significantly fewer capsular contractures with the underneath incision. Why? Because bacteria appear to be the likely culprit in causing capsular contracture, and the nipple incision exposes the implants to more bacteria.
And undoubtedly, patients have more swelling and pain with the nipple incision.
Lots of good surgeons use that incision with great results. I do when patients ask for it, too. But as time goes on, I'm going to predict that more and more studies will validate that study I referred to earlier about capsular contracture through the areolar incision.

There are a few issues to consider here.
While I have performed this procedure without difficulty there are a few concerns that have been raised by surgeons regarding this approach:
An implant of this size requires an incision that is typically a minimum of 4 cm and preferably up to 6 cm to minimize trauma to the implant and the possibility of "fracture". Is your areola large enough to accomodated this size of an incision?
Other surgeons believe that there is a higher potential for bacterial colonization which may perhaps lead to biofilm formation and increased risk for capsular contracture. To the best of my knowledge, this has not been proven but is a concern.
The use of an areolar incision may be more advantageous for darker complected individuals due to lower risk for keloid scar formation around the areola in comparison to the crease incision.

I would say it is a periareolar incision, where we place the incision just at the junction of the breast skin and the dark pigmented areola. It all depends on how big your areola are. If you have very small areaolas, then you may need to have the implant inserted via an inframammary incision.
Hope that helps.