Trying to choose the appropriate size of kayak from all the styles, lengths and models out there could be a bit daunting at first. It can be a lot simpler if you know precisely what factors to look at before making a decision. Aside from the type of waters you will be navigating, a basic rule is to choose your kayak as you choose your shoes: It must fit you well, be comfortable, serve its purpose and you should try it out before purchasing it.
Finding a kayak that suits you well
To find the right kayak size for you, first check its suggested paddler weight range, since there is a general relationship between body weight and height, when it comes to design. This will help you narrow your search. Look for those kayaks where your body weight falls within the suggested range, When your weight falls within the ideal loading, the kayak does not submerge further than the designed waterline (DWL), and so it will navigate in its optimal performance. Having narrowed your search, you should then look at the front height of the kayak's cockpit. The higher the cockpit, the easier it will be for you to enter and exit your kayak. Also, the higher the cockpit, the less water you will get on your lap, and the less water going into your kayak.
Once you have found a couple of kayaks that meet these requirements, try them on dry land. It should feel comfortable. You should be able to stretch your legs, move your knees and change seat positions comfortably. If you plan on using your kayak for touring or long trips you might want to look at the different measurements of cockpit's rear height. This will allow you to have a back rest when needed.
Choosing the length and type of kayak
You can broadly narrow your search for length and type of kayak by knowing, first, the types of waters in which you will be primarily using your kayak: lakes, ocean, small rivers, whitewater or all of them. Second, the intended usage of your kayak: for an expedition, day trip, touring, fishing, multi-purpose; and third, you should consider your skill level.
Generally speaking, longer kayaks tend to be narrower and so can navigate faster, yet they are not as stable, and you should be proficient at maneuvering these. Shorter and wider kayaks are more stable, yet they navigate slower. If starting, this could be the best option. If you will be navigating in turbulent waters you will need more control to handle your kayak stir when needed. If so, you must make sure the kayak you purchase has knee braces. Make sure they fit comfortably.