There are actually two main reasons for snorkel tube’s length the first of which pertains to what is called dead air space. When a person breathes in normally, they take air into their lungs where it is utilized by the body in a process called respiration. The purpose is to take oxygen from the air and through a physical and a chemical process and convey that oxygen to the person's tissues and cells. Carbon dioxide which is the result of the metabolized oxygen is expelled as a person exhales. The actual transfer of gasses only takes place in the lungs but they are not the only spaces where air is located. The sinus cavities, mouth, wind-pipe and nasal passage also get filled with air upon inhalation but the air in these areas does not get metabolized or used. These areas are what is termed as "dead air spaces. A person's lungs are sufficiently equipped to bring enough air in past these dead air spaces to get the supply that is needed in order to function properly.
When in use, a snorkel adds another dead air space for the body to deal with. While having a longer tube may sound inviting, one must realize that the carbon dioxide gas that is expelled during the exhalation process goes out the same way it comes in traveling through the same dead air spaces that were along the path during the inhalation. The body does not get rid of all of the carbon dioxide during the exhalation as these dead air space are holding on to some of it. The human lungs are still able to compensate for this minor reintroduction of carbon dioxide back into them during inhalation. It can also tolerate the additional dead air space that provides that the snorkel is not too long. Today they are made so that the body can still get enough fresh air into the lungs by breathing past these air spaces.
It has been determined that the maximum usable length is limited to 16 inches (40cm) which takes in the second reason for limiting it's length which has to do with physics and the properties of pressure and depth relationships and their effect on air spaces in general. Rather than getting into complex and confusing mathematical equations, it is better to try to explain this in simpler terms.
The deeper in water that a person goes, the more pressure the more pressure is placed on them due to the very weight of the water itself. This pressure pushes on the air spaces causing them to compress. The lungs are an air space that is affected by this. The muscles that are involved within the human body that are used to inflate the lungs would not be strong enough to bring air down during an inhalation at a depth any greater than 2 feet (61 centimeters) below the water's surface. This information is based on the average human adult in relatively good health. The diameter of the tube itself also is also designed for reduced resistance during inhalation. Measuring .75 inches in diameter (20 millimeters) the length and diameter ensure that the air needs of the user are adequately met in a manner that is close the effort used in natural inhalation and exhalation.
With this said it should also be noted the snorkels designed for children are both shorter and have a smaller diameter bore size than those of the adult variety measuring 14 inches in length with a tube diameter of .5 inches. This is based on the smaller lung size of children. Children should not be encouraged to use adult styles as the ability to breathe past dead air spaces is greatly diminished.