Straight vs. Shaped
Early skis were very long and very straight. As advancements were made in the construction of skis, however, shaped skis were introduced. The shape of shaped skis allow more surface area in a shorter ski than straight skis, so the desirable length of shaped skis is much shorter than straight skis.
In general, the heavier or taller you are the longer your skis should be. The extra length allows the skis to support a heavier or taller frame without problems.
Skier ability should be taken into account when choosing ski length. Beginners will find it easier to learn on a short ski, while advanced skiers may enjoy a bit more speed on a longer ski.
Skier style should also be taken into account. If a skier prefers to ski short quick turns, a shorter ski will suit her better. If the skier is looking for speed, however, a longer ski will be the better choice.
The terrain a skier wishes to ski is also important when choosing ski length. For moguls and trees, a short ski is preferable. When looking for speed or skiing deep powder, a longer ski is best.