Blimey!! How about starting with a spidergram and using all the different headings to start with, so
Stress
Diseases
Accommodation
Breeds
Coats
etc etc
And then write down as many things as you know about or ideas you have under each section, use any pictures you have to help, keywords or write in detail. Whatever - just get it out of your mind and on to paper.
Then maybe that will trigger ideas for what you want to write about, your own experiences/knowledge, your own rabbits, rabbits you have seen on here and the experiences others have talked about, any books you have, online resources.
I am guessing it is going to have to be quite summarised under each section to get it all on to a poster that people can actually read. Photos or pictures (magazine cutouts?) may help?