Hi Diann,
What a thoughtful and creative grandma you are. I'm sure those kids will love their special gifts.
Disney autograph books come in a variety of sizes and styles. They're sold in almost every store at Walt Disney World, as well as on DisneyStore.com. The exact offerings change from time to time, but this
4.5" x 6" book is quite typical and would probably be considered the traditional standard. There are also
versions which are a bit larger to include photographs and an attached pen.
You should also know that there is no requirement that characters sign an official Disney autograph book. If you'd prefer a different style book, that's fine. Characters will also sign almost anything that's G-rated and not currently being worn. I've seen guests get hats, t-shirts, calendars, photo mattes, and dozens of other items signed.
Have a wonderful visit to the parks!