Snowboards come in all different shapes and sizes. The type you choose will basically depend on your riding preference and standard. There are essentially three main types of snowboard: alpine, freestyle and freeride.
Freestyle boards are designed for doing tricks and riding half-pipes (they are designed to be used with 'soft' boots). They are very flexible and completely symmetrical/multi-directional so that they ride backwards (fakie) as well as they ride forwards - thanks to their twin-tip symmetry. Although these were all the rage a couple of years ago, especially with the skate board generation, the trend has swung towards freeride boards.
Freeride boards are designed for all-mountain, all-terrain use with soft boots. Most of today's freeride boards are directional (asymmetrical) - they ride better forwards than backwards since most riders spend at least 70 per cent of their time riding forwards. That isn't to say that you can't ride them properly fakie - They are all twin-tipped so tricks aren't a problem. The trend this year has been for longer boards than before, as people are using them more and more for all-mountain riding - carving on-piste and surfing powder off-piste.
Alpine 'carving' boards are designed for cruising and carving on the piste - fast. These boards are used with 'hard' boots - transferring your leg movement to your board with greater precision and control. Alpine or 'hard' boards, as they are often called, are much stiffer than the other types of board as they are not designed for jumping and landing jumps. They are designed to travel in only one direction - forwards - so they have only one tip and are flat at the back. They also tend to be longer and thinner than most freestyle and freeride boards. Alpine boards are rarely seen these days.
The size of board you choose depends on your height, weight and standard. There are a number of wider boards designed for riders with big feet as well.