It's so easy! Just pick a design, pick a shape, and pick a length.
The shape you choose will be scaled up or down to fit the length so the proportions stay the same and you get what you want.
The "Rock" is a fairly standard short board shape which is suitable for most people and conditions.
The "Gun" is super skinny for those who prefer Kate Moss to Kim Kardashian.
The "Fish" is a nice meaty fish shape with a swallow tail perfect for fat waves and having heaps of fun.
The "Slim" is a skinnier version of the "Rock".