Size & Cross Section
Longer kayaks tend to be faster and track - go in a straight line - better than shorter ones. Shorter kayaks, while not as fast, are much more maneuverable and tend to be lighter and easier to handle. Longer kayaks are better suited for cruising and touring on open water, but are a little limited in tighter areas. A shorter kayak would be a great choice for exploring estuaries or small lakes and rivers, but probably won't have enough gear space or speed for longer day trips or overnight outings.
The width, or beam, of a kayak is measured at the widest part, usually the middle of the craft. Generally speaking, the wider the kayak the more stable it is. With added width you tend to sacrifice speed. A wide boat would better suit someone taking pictures, bird watching or fishing, while a narrower kayak would be a good choice for those who want to cover greater distances with efficiency and speed.
The cross section refers to the shape of the sides and the bottom of the kayak. How the bottom is constructed plays a big part in the stability of the kayak.
Flat bottom kayaks are very stable in calmer waters but become less comfortable as the waves get larger.
Shallow arch bottoms feel initially less stable but tend to be faster and are better behaved in unsettled water.
V-bottoms improve tracking but may have less initial stability.
The sides of the kayak also have importance. Many wider kayaks have a tumblehome design, meaning the sides actually curve inward as they come up creating narrower beam on the deck. This enables the paddler to more easily reach the water while still having the stability of a wider kayak.
Other kayaks have flared sides, meaning the beam increases the deeper it sits in the water, providing more stability as the cargo weight increases. This design provides increased stability when the kayak is on its side or in unsettled water. Kayaks can also have straight sides, which are a compromise between the two.
The diagram below illustrates the basic features of a typical Old Town kayak design.