Currently it's a 8 Mb max.
That's a rare option that may be useful if one needs to create a small torrent file (some trackers limit size of uploaded torrent files) in case data size is huge (100+ Gb) but number of files is small. For ex. collection of videos.
As I understand most Bittorent clients support 16Mb chunks. But not many allow to create such torrent files. Also we have a superfast internet nowadays.
Once I considered creation of a 200Gb+ torrent with e-books. And had to search specialized 3rd party tool ("Torrent Creator") to try 16 Mb chunks. In the end it doesn't help as I had too many filenames in torrent file anyway. But at least I tried.
Also I've found only 1 torrent in my 400+ torrents collection with piece size 16Mb. It's a 1.2+ Tb video collection that i found throw DHT search engine.
Still I'm suggesting to let users create torrents with 16 Mb size of piece. Those who have small number of files in torrent will benefit. Others will make sure it doesn't help to decrease size of torrent file without googling 3rd party tools for that.