I think it depends on the purpose of the portfolio. If it is to be carried around and shown for professional reasons, a standerd smaller size would be OK. If you are thinking of a presentation to be displayed, a uniform presentation in 20x16 frames (or mounts) has more impact. It is nice to see and I suppose to arrange, a display of constant size irrespective of the size of the image which would itself determine whether it would be landscape or portrait hung.