wow that is a huge undertaking. One I don't think I could do!
Have you thought about maybe doing the online digital books? Not as personal but easier and probably cheaper in the long run. Or what about a dvd slide show? Great programs out there you can personalize it.
If you have your heart set on a scrap book I would go definately with the smaller book size. Use 5x7 of the group shots on one side and then maybe a few small pictures on the other.
I would also suggest to pick a color theme that is easy and not deal with a ton of paper paper. Maybe red, black, yellow and white. Or break it down by parks for colors. I saw one album done this way and it was stunning. Epcot was grey colors, MGM was dark blue and white, Animal Kingdon was browns and magic kingdom was traditional mickey colors. Keep embellishments simple to cut down on cost. have the pictures and journalling be your focus not tons of stickers and other expensive items. Ribbon is also a good choice it can be used as borders, accents to photo frames, etc....
Good luck! Have a wonderful trip!