How important is a strong ending

Do not put weak images in the back of the book, ever. Leave us with the memory of a great image; do not make us turn back to the cover image because the last image cast so much doubt on your whole portfolio that we suddenly feel like we were wrong to be optimistic based on the front of your book.
What is the appropriate size for a portfolio? How much does it vary depending on purpose?
Not so huge that handling it is a back breaker, but big enough to do your work justice. Print size is important; making a choice about the edges of the pages is also critical. Look at well designed photo books you will see a lot of either a full bleed blast of borderless imagery on a big page, or a huge amount of white space around a small image. Are your images loud or soft? Gentle or brash? Make the book size and design consistent with the content.