For my display portfolio, I was told that larger is better. So after a few tests I did by showing different sizes to people and getting feedback, I felt the ideal size was 11X14 in a 16x20 portfolio case and matted 16x20. I use flat-black double mats and mount the images with adhesive (non-foam core) board. I can easily pull the images out of the sleeves in the case and display them in the open as well as show the images book-like out of the portfolio.
I tried 8x10 and it just wasn't as impressive as the 11x14. I tried 16x20 and it was too unwieldy to carry around easily.