Ok ive been told its best to get a already put together board as your first, would you guys recommend one in particular? And what difference does deck size make?
I wouldn't get a board that's already been put together as my first unless its at a legit skate shop, not places like Big 5, Sports Authority, Wal Mart.
As for recommendations, I say you build your own custom complete at a local shop. You're good with almost any board there. Since you're just staring out, I'd say you can't go wrong with a Girl,Plan B, Baker, Real, Sk8Mafia... Almost boards are good too but their tails are soo small.
Wheels: Bones STF
Trucks: can't go wrong with Ventures, Thunders or Independents. (Ventures and Thunders are my personal choices).
With a setup like that you'll have a fast, durable, and long lasting board to start off with.
Deck Size is all personal preference. Im a shoe size 9-9.5 and I like 7.75. I just recently got an 8.0 and i love it for park skating, rails... But not as much for tricks because i have to get used to it
If you just wanna buy a pre-assembled complete, eBay has some good ones with better setups than any completes I've seen.