Length is everyone's favorite dimension. A little longer, a little shorter, we're all one change away from the perfect boat. Length is also a major ingredient in the formula for speed. The longer a boat is, the faster it can travel before it overtakes it's own bow wave and tries to climb over it. This is called reaching hull speed. For displacement hulls, especially low powered ones like kayaks, this is the practical upper limit for speed.
The idea that longer is better seems widely held right now. There is no question that for some situations longer boats are preferred: Expedition type trips, long crossings, any time speed and/or volume are valuable.
Don't overlook the advantages of a shorter boat too. Long boats can be at a real disadvantage for playing in waves, and rock gardens, and they offer a longer lever arm for the wind when it really wants to grab you. Weight and cost both climb as length increases.
Long boats are said to be more directionally stable, which is only partially true Length alone will not guarantee good tracking, but the longer your boat gets, the harder it will be to manuver. Manuverability is not incompatible with good tracking, and it makes a better boat, one that is more fun to paddle. The ability to make small course corrections underway is a necessary function in a useful cruising boat. The overwhelming majority of historical kayaks did not need rudders to do this.
I would call a kayak "long" after the 17 foot mark. 14 to 15 feet is a pretty standard size, and I would call a boat short at around 13 feet, but have built kayaks as small as 8 to 10 feet long.
The beam, or width of a kayak, has more to do with it's horizontal stability than almost any other factor. A basic rule of thumb is if you want a boat to be more stable, make it wider. Like any quick fix, this can lead to problems when over zealously applied.
Kayaks can be too beamy. Unless you are rather tall, your paddle is likely to bump the cockpit or deck of a wide boat. 30 to 32 inches is about the limit to avoid this. A wide boat can have handling difficulties too, in leaning for turns, and in rough water.
Kayak stability is worth a deeper look. Kayaks are not very stable platforms when compared to many other type of watercraft, nor are they meant to be. Initial stability in smooth water is very different from stability in rough water, and in trying to make a kayak what it is not, you can lose what it already is in the first place. A wide, initially-stable flat- bottomed kayak can be a different beast among the waves. Since the same force that acts to create initial stability in flat water is what tries to capsize you on the face of a wave, you may end up with the opposite effect you were looking for.
Be careful though. Numbers have that seductive, pure reason appeal, but they can be misleading if not leavened with some practical experience. I build a 17 footer at a 23 inch beam. The beam divided by the length gives a ratio of .113, just over 11%. It's a nice boat, so I know the L/B ratio is good - for that boat
I also build a 14 plus footer witht a 25 inch beam. The beam,( 25) divided by the length (172, in inches) gives a ratio of .145 . This boat is a good one too, so it's ratio must be OK. The difference in length between the boats is less than 3 feet. The ratios are only .032 apart, which doesn't seem like a big change. And yet the boats are really quite different.
What this tells me, is that for kayaks, the ideal L/B ratio for a given boat is pretty specific to that boat. You might use one ratio for all 17 footers, but you better have another one for the 15 footers too. Likewise one for the racer, and another for the cargo carrier.
There is one other interesting thing about the difference in these two boats. There is some discussion about the ideal beam. 22 to 23 inches is pretty popular right now, and a 25 inch beam may seem rather wide. But the fact is that a shorter boat, 13- 14 feet with fine ends, is liable to have proportionally less wetted surface than a narrower, but longer and fuller- ended boat.
Volume relates to carrying capacity. Kayaks are divided into high and low volume hulls. The Aleut or Greenland boats are good examples of low volume hulls. The Nunivak and Bering Sea types are higher volume hulls.
A typical depth amidships for a low volume boat, such as a baidarka is 7 to 8 inches. A higher volume boat, say a Bering Sea type, will have a depth closer to 12 inches. Remember that you need to include deck height for a total depth. Low decked, low volume boats add 2 to 3 inches more for deckheight, and 4-5 inches for a hi volume boat.
Kayaks are sometimes classified according to their total volume, often measured in gallons. This is a useful indicator, but remember that total volume is divided into immersed area, what the boat actually floats on, and her reserve buoyancy, or that volume above the LWL (Load Water Line). Naturally, there is a constant interchange between these categories, depending on loading and state of the sea.
Excessive volume in a kayak can result in a boat with little immersed area and a lot of freeboad, not a good combination for wind and waves. Getting the right volume and freeboard for your boat is an important part of both the design or modification process.
Tailoring volume can be done by adjusting beam, depth amidships, sectional shape, and the use or non-use of deadrise. Further discussion of these points are found in the posting about midships sections.