Generally I see riders picking up a bigger size of the board, than they should. A bigger board has more forgiveness and makes sense for very-very beginning level when you just learn how to follow the boat (up to your first ever trick).
Then a bigger board gives you less control and demands more power for any maneuver. If you plan to buy your own board, I believe you plan to go further than riding straight with some carving moves.
Put attention on the size of the board you choose, so it will HELP you to progress, not the opposite.