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Check out this page knads. You can use it for a size comparision, and it should help you figure out what you want. Feed prey that is the same diameter around as your snake at it's thickest point, or go a little bigger. From the looks of your picture, your snake could take a frozen/thawed/warmed fuzzy rat.
As per your substrate (the "stuff" at the bottom of the tank) question, I would recommend newpaper for the first few months because it is easy to maintain, you can check for the presence of external parasites (mites), and you can easily collect a fecal (poop) sample that you need to take to the vet so they can check for internal parasites in your snake. If you haven't located a vet in your area yet, try the links in my signature. One of them should be able to help you out.