The most important part for fitting a Soft blazer is in the Shoulders. Make sure the seam hits right at the end of your shoulder, too much extra fabric will bag out due to lack of padding. If it is too baggy or too tight it will not look good. The next important aspect is going with a medium width notch lapel. This works best with a soft blazer because it doesn't overwhelm the unstructured shape. The top button of a two-button blazer should hit more than a bit above the navel and never ever below the navel. And last the sleeves should hit between your first thumb joint and your wrist. And, as with all jackets, make sure to get them tailored so a quarter inch of shirt cuff falls below your jacket.