Check the height. Your bench should not be too high or low. When you reach your hands out to the piano, your arms should be parallel to the floor. Adjust your seat, or sit on a book or two if necessary.
Distance is important. Your bench is too close to the piano if your arms are being pushed in towards your body. It is too far away if you need to stretch your arms out fully to reach the piano. The correct distance allows you to reach the keys comfortably. Remember your arms should be parallel to the floor.