I think a portfolio would be better when it is in a binder. Something that had holes binding it together and not just a plastic clip, because everything will end up on the floor when they try to look at it.
I don't know if there is a particular format but I like to have my portfolio prints on 100 to 150 weight card stock. A 0.5 white border is not a bad idea either when printing. I ussualy go with 8.5x11 format for eay transport.