Your body frame is one factor to consider in determining boat length; you want to be comfortable, especially in a sit-inside with a cockpit (note: with adjustable footrests, many small kayaks can still be paddled by taller people; conversely, shorter people can also paddle longer touring kayaks). But hull length also comes into play for performance. In general, the longer the hull the faster it is, making it more efficient on longer paddles and crossings, but the harder it is to turn.