Well, I spelled it right in the edited version ;-))

I think it's called bitesize because most of the fics written for it are rather short, but you can go longer if you want to. The maintainers post a specific challenge at some point (usually at the beginning of a hiatus), for which everyone can post prompts, and then everyone can write ficlets for these prompts.

Alternatively, I guess one could simply post a comment fic challenge over at bones_ga - I've never done one for this fandom, but in my old fandom, some people would host such challenges on a regular basis, and they were a lot of fun.