It doesn't look like it should be a problem right off, but I would suspect around 15-20 prints would be the maximum per portfolio, considering that it has to come around with a certain frequency to sustain a conversation about its contents.
Matted prints lend themselves better to a box-type portfolio, where the matted prints are simply stacked. Unmounted prints would work better with a loose-leaf type portfolio, with black pages and acetate sheaths. We might want to agree on what sort of presentation we prefer. I have no strong preference either way.
If we go with matted prints, each contributor could print out an information sheet and tack it to the back of the mount. If we go with unmounted prints, the info sheet could go on the backside of the portfolio page.
I'd lean toward 8x10" as a maximum size, but lament that that would exclude submissions of contact prints from ULF negs. We could go with an 11x14" portfolio and just say that if contributors want to include an 11x14" contact print, it will have to be flush mounted. (Of course, then I'll really want to see one of William Levitt's 12x15's from his new camera, and one of Carl Weese's 7x17's, and...anyone shooting 20x24" around here?).
The call for entries would certainly be a necessity.