Rider Weight: A board has a measureable camber or bow throughout the center. This camber is responsible for giving the board "pop" and liveliness. When a snowboard's camber is greatly diminished or gone, it is time to get a new board as it likely has become a good bit more flexible as well . Camber generally gets higher as a board gets longer in length. This is because of the simple fact that a longer object is easier to bend than a shorter object, and to accomodate for the extra weight of a heavier rider on the longer board. The manufacturers may also adjust the camber in relation to the stiffness of the board's longitudinal flex.
Boot Size: Boot size can play a huge role in riding performance. At the rider's given binding angles, 1/2"-3/4" is the desired overhang of the boot's toe and heel along the side edges. If the boot does not reach the edges, the rider is unable to turn well because of a lack of edge leverage. If the boot's toe and heel overhang off the board too much, the rider will experience his or her boots noticeably dragging through the snow, which will also make turning difficult. Some riders can get away with a little more than 3/4" overhang depending on how far they get on edge and the type of snow they are riding on. If a rider needs to go to a wider board the length may be slightly lowered in certain circumstances. The lower length can offset the added width and thus help to even out the actual board mass in contact with the snow.
Rider Style: While there are several different types and combinations of riding, the two most popular and easily differentiated are freeride and freestyle. A freerider is characterized as a boarder that likes, but is not limited to, ripping down the mountain in search of that perfect carve. This type of rider usually opts for a longer board to provide extra speed and edge hold. A freestyler is characterized as a boarder who prefers, but is not limited to, getting air in the park/pipe in hopes of nailing that perfect jump. This type of rider generally likes a shorter board to provide less swing weight while rotating in the air.
Type of Snow: The type of snow a rider normally is on will also affect the length of board he or she may choose. Many boarders prefer to go an extra 4-9cm or more in length when riding powder as opposed to icy or packed snow. The additional length will provide extra speed and better flotation through the softer powder.