I'm a big believer in ergonomics, had to be as a production typist back in the day. QuiltE is correct with the basics. I have a long torso and short little arms and legs, not as bad as the alligator in the Geico ad but close enough
For me the typing trays on most office desks are too low, table height is the correct height for me.
However, I am too used to my machine being on top of a table rather than set in at the correct height in a sewing table. I just feel awkward at that level. I could probably get used to it and maybe enjoy it in time, but so far I am content with a standard height table. I have started looking for a good table at a good deal on Craig's List, but I'm cheap and I might have to be looking for quite awhile.
I do have an adjustable office chair for my sewing room. Even if I am reaching up a bit with my short little arms, my feet/legs are correct, my foot pedal is at easy reach.
Another thing to keep in mind is to change your position every half hour or so. For me while quilting that means that I will sew for about half an hour, then I get up and walk to my ironing board and iron while standing for a bit. I actually move my ironing board out of my sewing room and into the living room just for a bit more change/movement. But there is that big part of me that just loves to stay sitting in my chair with my desk behind me, my sewing table in front of me and my ironing board lowered to sitting position...