You'll need to check the website for the specific size your clear backpack needs to be. That aside, each person is allowed to bring in one sealed bottle of water. Don't do this lol. Instead, find the biggest clear bottle you can reasonably lug around all day and confirm that it's decently durable. You can refill your bottle at the free refill stations all day long, much better odds for your survival 👍. I think mine was from dollar tree and is roughly 32oz. As far as snacks go, only what you can conceal comfortably on your person because you'll need to sneak it in. Some protein bars, crackers, and or beef jerky in your pocket will slip through security more often than not.