How long should a brief CV for a call for papers be? I have seen in some call for papers for conferences request a brief CV together with an abstract. 

Most call for papers I've seen in the Humanities (my field is history) usually look for a 250-300 word abstract.

Slightly a side note, but when a PhD student or Early Career researcher submitting that may not have much to put in a CV, is there a different format for a "brief CV" or not?

I am wondering what is expected in the "brief CV" requested and how long it is expected to be?
If it is more than a page it is too long. As for what should be there, say something about your education, current employment and some of your publications. It isn't necessary to be comprehensive but include things that are most relevant to the conference and to the paper you are submitting. 

That is probably about enough, but if it seems too short, include honors, etc. if you like. 

All the committee is looking for at this point is that you are qualified to say what you want to say in the resulting paper. Focus on that above all.