By far the number one issue people have with body use is sitting at a desk, with or without a computer. It’s not that it is hard to do; the problem is that we spend so much time doing something our bodies aren't prepared to do for extednded periods of time.
The two questions I am asked the most regarding ergonomics are: What size exercise ball is correct for me? (it really is!) And what is the best way to sit at a desk?
The answers are similar, believe it or not. The first thing we need to keep in mind, however, is that it is not always possible to control ALL factors with regard to body use and work. No matter what the label says on the tool that you are using, that tool may not be the most suitable for you! That is everything from a kitchen knife, a hammer, an exercise ball or an office chair. The only way to know for sure that a tool is correct for you is to know how you are supposed to be using your body while using the tool and then to find the tool that allows for the appropriate, good body-use pattern.
With that said and with regard to the ball, the correct size ball is the one that when you are sitting on it, with your lower legs and back, both plumb to the floor has your thighs parallel to the ground. Your knees and hips should both be at about a 90° angle. ALL balls, if inflated to their stated size, will deflate over time, so please pay attention to this and manage it accordingly!
With regard to a chair and desk environment, first, you must be sitting on your chair correctly, and then a desk can be sized for you. If you are of very short stature, this is very difficult. The rules for sitting on a chair are the same as those for sitting on the ball. With back and lower legs plumb to the floor, your thighs should be parallel to the ground. Chairs are easy to adjust to accommodate this for most people. The seat base height can easily be adjusted.
Now, the interesting stuff. While the seat base depth is not adjustable, for and aft positioning of the base often is, and two things need to be considered: 1) Does the front of the base allow your legs to hang in a plumb manner and 2) while seated on the chair base, is the small of your back up against the lumbar area of the chair back? We all need to consider the depth of the seat base. If you have short thighs, a long seat base is not appropriate for you.
Chair-back height is adjustable on many office chairs. Proper height is such that the lumbar portion of the chair back is situated comfortably in the small of your back. When the back, base and height of the chair are adjusted correctly for you and you are seated on the chair, your feet should be flat on the ground, lower legs and back plumb to the floor, thighs are parallel to the floor, your back should be gently supported and you should be able to look directly in front of you, eyes looking generally parallel to the ground. This is just for the chair!
IF the world were really a nice place, not only would our chairs be adjustable, but also our desktop heights. But desks top heights are not adjustable. There needs to be a good relationship between your arms and your work surface and its tools, whether it be a computer keyboard and mouse or pencil and paper. When seated at the desk in a ready-to-work position, your arms should be parallel to the ground. Most desktops with a keyboard on them are too high and the user should be using a pullout drawer or drop for the keyboard. Placing the keyboard on top of the desk usually will not work as it is too high. If you need to raise your seat base up so that your arms are parallel, then your feet and legs will no longer be optimally oriented. If this is the case, use some sort of small stool to place your feet on. I have some clients that use a stool that is originally intended for use by guitarists. It is relatively cheap, and it’s small and they are often adjustable.