Well if I knew the answer to that question I would tell you. It is something I have been struggling with myself. I end up gathering folders full of research, which does really lead me to my final product, but you obviously cant show all of this can you? I think extracting the key pieces of information that really influenced your design would be a start. It really is hard in my opinion to show research without typing a lot of text. So sorry I can't help you on that front... let me know when you figure it out though
As for the amount of pages in your portfolio.... For a hard copy that you would take to an interview, I was told keeping it consistent is key so that it can flow nicely. For example if you display it as a landscape book format, you may have a brief introduction page followed by 3-4 project pages, but make it so your projects always start on the same side of the page I think. It is hard to condense big projects down to a couple of pages, but I think your better/bigger projects would be allowed an extra couple of pages. It depends how and where your portfolio is. My website layout of portfolio isnt the same as my hard copy, because it suits the website in a different way.
This is what I was told at University anyway, hope it helps