Whatever is after the weaned stage under 100g.
I've never been bitten, so I wouldn't know. My rats live in my bedroom and I play with them often, so they're well socialized.
I'd also like to add to the live debate, socialized rats are less prone to bite than unsocialized ones. They tend to be less frightened when introduced to new situations (including being put into a snake enclosure). Frightened animals will attack and fight back...
I apologize that I offended you.
However, mice are known to be bite aggressive ... Are you feeding adult mice? I wouldn't put it past some mice to bite anything that comes near it, humans included.
Behavioral studies have even shown that mice are more aggressive than rats.