Average grown up female nipple size be about 4mm diameter.
In my case - now looking at it, they are about that - BUT they do not project anywhere near as much 3 to 4mm, and that I guess be a problem.
Too little flesh in front of the piercing then.
Since I'm quite sensitive – still, after 1 1/2 years HRT, to now put a piercing... no THANK YOU!!!
YMMV... oh yes!
There is good pain and bad pain and THAT be bad pain,
Axélle