What size skateboard deck do i need to get?

Well, it depends how big you are, but when I was smaller and younger, I always skated mini decks, around 7.25." Element makes a lot of minis, but you'll probably have to get one online. A lot of skate shops don't carry them. Otherwise, you could just go with a 7.5" deck, which are much more common. Either way would probably be okay.