In how much detail should one describe their past and projected future work for a junior scientist application to CNRS?
It may be dependent on the field and even on who is in the jury. In particular, the jury changes every few years and they decide by themselves what procedure they will follow to judge the applications. 

Two of my colleagues who have been on the CNRS jury in different STEM sections in different years told me the following:

Your file will probably be assigned to one or two of the jury members who know most about the subject of your proposal. They will have an in-depth look at it and initially defend it on your behalf if they think it is worth it. These assigned jury members are the only ones that have to read your file. The others can choose to do so if they wish.

Besides this I can tell you the following from applying for a CR2 position myself (several years ago):


I described my past research, point-wise per university where I worked. This was 2 pages in total.
I described my research proposal in 11 pages. It included figures, preliminary data and references (much like a research paper). I described a short-term (5 years) research goal as well the many possibilities to expand the research later on.


This must have been reasonable because I was invited for an interview. Of course there are surely many other ways to do it that may be equally good or better, but all I can say is what I know, and wish you good luck (you will need it. It is well known in France that the jury often wants to fund around 20 equally good proposals out of ~200 applications, but they only have the money to fund a couple).