The principle behind the various sizes is that the wider and thicker the board, the more stable it is in the water. The rule of thumb for beginners and if you are already an adult is that you need to consider applying for one that is 1 to 2 feet taller than your standing height. For kids, 6 to 18 inches taller is basically sufficient when they are lighter and as a consequence, more buoyant. You can buy kayak parts from many online shops.
These boards are for the advanced surfers. They are small and delicate. Short boards are designed to turn quickly and move fast around the waves. They are very difficult to stand up on and need a lot of balance to use. These boards come in an array of sizes and are relative to the size and weight of the person using it. The shape of each board also changes depending on what the surfer desires. You can have a square, flat, swallow-tailed and rounded back ends, all make the board move slightly different in the water. None are better than another, they are simply different.
Long boards are not necessarily for beginners. In fact, they are for intermediate to advanced surfers who can handle the big size and want a fun and easy board for small waves.