I think 4x5 or 5x7 would be ideal but even strips of 120 could work as well. A print doesn't have to be huge to see how the process itself is working for the printer. If someone wanted to go larger that would be fine but keeping it simple and easy for shipping makes sense to me.
As far as which processes to choose, how about everyone who's interested just say what they would like to see. Allot would depend on how many people are really interested. We could have a group for just Pt/Pd another for Cyanotype etc etc. Or we could mix it up. The important thing to me is to let someone starting out in alt process see first hand how prints from someone else look. So Nige, I think the rank beginer is exactly who this is for.