Well perhaps there is no single "right" way to prepare a portfolio. But here are a few suggestions:
Traditionally, keep all your sheets of paper the same size. The image size and dimensions can and do vary, as it is not uncommon to crop images or use different camera formats in the same portfolio. However, by having the same paper size you bring uniformity to the portfolio. On another note, it’s probably not a good idea to make your client keep turning your portfolio around in order to view the photos. If you cannot crop your images so that they have the same orientation; instead perhaps create two portfolios, one in portrait format and one in landscape format….or at least two sections.
The print sizes should be no smaller than 8X10 and no larger than 11x14. This is for ease of viewing (you want your work large enough to be seen), shipping (you want to be able to overnight it without costing a small fortune) and carrying (you want to be able to get around town without causing any accidents). Try to imagine going to your interview carrying a gargantuan sized portfolio, getting into the elevator, hitting people in the head as you walk through small corridors and so on. Not a pretty sight.
Another thing to remember is that printing 'full bleeds' (no borders) is not recommended - most clients prefer a small white border 1/2" minimum around the edges. This will help the photos stand out and make it easier to place them into those protective sleeves without getting fingerprints on them.
Finally, when setting up your portfolio remember that you will have two images showing at the same time; left page and right page. Always strive to choose images that play off of each other such as a similar concept or color tones.
There are many sources for portfolio boxes, books and cases. Here are a few that you may want to check out:
House of Portfolios
Light Impressions
B&H Photo
More Tips How to Create a Portfolio of your Work