If you were to ask a craftsman what the best tool is, his likely response would be “for what?”. Just as the selection of a tool is contingent upon the task to be performed, selecting a chair should be done with the task to be accomplished and user in mind. If you are working on a computer, you will likely be adopting a different posture than if you are poring over physical documents. Secondarily, selecting a chair which complements your particular physical peculiarities will help you to remain comfortable and healthy regardless of the task at hand.
Most products are designed for users who fit between the 5th and 95th percentile. If you are outside of these percentiles, care should be taken to ensure that your chair will be safe for you to use. For example, a woman who is beneath or near the 5th percentile for height will be unlikely to fit properly in most chairs, even once they are adjusted. A seat which is too deep or high can cause circulatory problems in the legs. While ideally a chair which fits properly will be employed, short term fixes could include a foot or back rest which decreases the effective height and depth of the seat.
Additionally, seat width can be a problem. Armrests which are too wide mean that the occupant of a seat will be denied the benefits of proper support. Arm rests are an integral part of proper back, head, and neck support, so even if the rest of the chair is adequate for the user and task, improperly spaced armrests can be a deal breaker.
On the other side of the spectrum, tall or heavier people will need to find chairs which match their own unique set of physical circumstances. Aside from the obvious need for their chairs to be able to support their weight, those who are taller are also at greater risk for back pain. Circulatory problems can also abound, as limbs far from the heart present a greater vascular challenge. Taller people should ensure that their chairs elevate them far enough from the ground that their knees are not bent over 90 degrees. Additionally, ensure that the arm rests are positioned in places which reflect the additional wingspan of larger individuals.
Taking the time to ensure that your chair matches your body is a surprisingly uncommon undertaking. Many people think that all chairs are simply uncomfortable, when in reality, they have just not found a chair that is right for them.