I have just over 7GB free on my 32GB iPad, mostly video and pictures. I have a fair amount of audiobooks on there, but I don't really use it for those, so they can come off anytime I need space. I do have all of my books, approaching 700 stored on there. They take up next to no space, under 700MB.
I have 9.8GB available on my 32GB iPhone. That is about half audiobooks, and I do use my phone for that a lot! I have all my books here too.
jdh - for the purpose you describe, you would probably be OK with a 16GB. I got the 32GB, just to play it safe. I have 5 movies, probably 2000 pictures, 39 audiobooks, 15 music CDs and almost 700 ebooks and still have room!