Kiteboards come in all different shapes and sizes. The strength of the wind along with the ability level and size/weight of the rider all come together to determine the right size board.
For beginners a large board is best, this allows maximum floatation and balance. Large boards are also useful in light wind, high wind speeds will allow you to ride a smaller board. The smaller and lighter the rider the smaller the board they will require. Likewise, the bigger and heavier the rider the larger the board they will require.
The style of riding you would like to do will also determine what board to buy. There are a variety of twin tips available great for just cruising along back and forth, where some boards, generally with more rocker are made for freestyle riding.
Surfing kiteboards are also manufactured for those interested in surfing the waves.