The most popular kiteboards are twin-tips.
These wake-style boards works in both directions so you don't have to ride toe-side which is an essential feature for beginners that can only ride heel-side at first. When riding heel side each foot takes a turn being the lead foot when kiting up and down the wind window.
Twin-tips are good all around boards for many kiteboarding conditions including the surf.
For wave-riders and people who spend their time chasing surf in the ocean you will want to own a directional, surf-style board as well. These kiteboards prevent you from riding with both feet forward so you need to be comfortable riding toe side when not kiting in the natural direction of your forward foot. We are seeing more and more people riding these boards without footstraps.
Kiteboard sizes vary and for many good reasons.
Wider and longer ones float better and will help new riders get up and stay up a lot more easily. Width is more important than length for newer riders and light winds. Board widths vary from 35 to 50 cm.
Longer boards are better in waves and chop as the added length keep it planing over vs. under the water. Average board lengths are 120 to 160 cm. Having the right kite size/board size combo makes a difference, especially when you are new. Until you can play with the kite strength and board ratio you need something that gets you up and riding easily.
For example, in the ocean you will often see kiters riding surf-style boards when the wind is light and the kite is small. The added buoyancy can help kiters stay powered up more easily.
Plan on doing some experimenting to find your best set-up.