When sitting in front of your machine, your arms should be bent at a 90 degree angle, when your palms are flat on the bed of your sewing machine. If they are not, then your chair is at the wrong height. You might need some assistance to figure out what that height needs to be, then measure from the floor to the top of the chair seat. That's the measurement you need to know when you go to buy a new chair. Take the tape measure with you to the store.
Over time chairs seem to loose their ability to stay the same height, then it's off to find another. This makes me kind of mad, since there never is anything else wrong with the chair, it just won't stay where I want it.