Thanks Victoria! I'm having an awesome time helping you plan your trip!
I dug out my son's Official WDW Autograph book that they sell at WDW to see how big the actual pages (not the cover) are since I know characters are very comfortable signing that size. The pages measure approximately 5 3/4" long & 4 1/4 " high. You may want to go a little bigger than a 3 x 5 - maybe a 4 x 6 index card - make sure to get the ones that don't have lines on them (just plain). I think its better to have a little excess than to run out of room. If the autographs don't take up the whole card, you can always cut it down to fit your needs.
You'll also want to have something sturdy to give it support for the characters while signing. You could use a small clipboard so that you could use it each time you have a character sign. Make sure you get a different color clipboard other than the color of your index cards which will help them see where to sign.
Choose a pen or Sharpie that won't smudge easily - Sharpies come in some really neat colors now. Some of the fur characters (i.e., Mickey, Stitch, Goofy, etc.) have an easier time with a bigger/thicker pen/marker. It can be tricky to hold thin pens with those big gloves on. Some characters use a stamp instead of signing their names (i.e, Buzz) so it may be helpful to him if you help hold the clipboard.
I answered a question for another Guest with some suggestions about character meet & greets for those guests wanting both pictures & autographs. Click
here to see that answer.
I'd also put your index cards in a plastic Ziploc to protect them in case it rains or from any spills that might occur during the day. As always, I'm wishing you many magical memories :-) Allison