Countless times have I seen people riding mountainboards which are too small for them or too big for them.
A great board for kiteboarding is not normally going to make a great downhill board.
The requirements are totally different.
A kite board should be short and light, and able to turn quickly and easily.
A downhill board requires a lot more stability, this means it will be heavier, longer and have much stiffer turning.
If your riding on small trails a brake is another consideration.
First things first, deciding on the purpose of the board.
Downhill use or kiteboarding are the main two so we will focus on those.
For kiteboarding you essentially need to decide on a couple of things in order to arrive at a compromise.
You can trade in stability at speed for a lighter and shorter board.
More advanced riders typically choose the lighter board, as their skill makes up for the lack of stability.
They lightness helps when the board is in the air and the shorter deck length reduces spin weight.
Downhill riders typically prefer a board with channel trucks as these are considerably more stable than skate style trucks.
Length is chosen to suit a riders height.
Ratchet bindings are preferred as they keep your feet attached more securely to the board.
The second major thing to consider is budget.
It has been seen over and over again that as soon as you get more into a sport and your level improves you need to upgrade your equipment.
A first experience on bad equipment can put you off for ever.
For this reason we all suggest that people spend as much as they can afford to.
Do not go out of your means however.
Your size is another important consideration.
A deck that is too heavy/long/big is no fun, and one that is not suitable for your weight will simply perform badly or worse, break.
Seek expert advice where possible.