Be ready for anything! My trainer had put a restriction on a couple of us because we are pretty tall (5.6-5.9") and I being 5.7" ended up with a 13.2 hh pony, I'm pretty sure my leg came down to his knees lol!
Be willing to be there awhile ;)
IEA is an amazing amazing thing to do because it teaches you to be a rider who can adapt and make the best of any situation. I would be sure to arrive early and watch all the horses warm up and watch all the horses go at least once. Once you know what horse you pulled make sure to watch a few more riders ride and see what works best. Be sure to look at the horse descriptions. It's an awesome opportunity and congrats to you for taking the challenge. All the best
I've been doing iea for 3 years now! Its so much fun, but my biggest tip would probably be to remain calm no matter what horse you pull!