My personal opinion is you should choose the size for each image based on the final selection of images you choose. I have also seen this advice from professional consultants. Debra Weiss is a consultant who helps photographers make their books and has offered that advice to many photographers over on www.EditorialPhoto.com where pro editorial photogs congregate. I recommend that organization for all US photographers here. It only costs $50 a year and pays for itself in all the knowledge you can gain from their message board and members-only content.

As for book vs portfolio artist folder (loose prints), a book is a really nice presentation format and keeps them all together. It looks very professional and tells the PE or AD that you are serious about what you do. It also shows them what your images may look like in their publication(s). The benefits to loose prints are that they allow a PE or AD to place specific images side-by-side and compare them to one another, they are easy for them to handle, and they make it easy for you to update your portfolio you as improve without having to reprint an entire bound book.

Will Hawkins made a book using iPhoto on his Mac before he went to NAMM earlier this year. He said the book was a big hit, and in fact some people wanted to buy copies of the book because they thought the book was for sale. That speaks well of a book presentation format. Maybe Will can provide more information on his experience at NAMM with his book.