The blazer should be tailored to fit the waist and not be too tight or too loose. The length of the blazer can vary depending on personal preference, but a classic length hits at the hip or slightly below.
When you're trying on a blazer, the shoulder pads should sit directly on your shoulders and lie flat. If they're hiked up or they sag, try a different size. Another indicator of good fit is that the seam where your blazer front meets the sleeve should align precisely with where your torso meets your arm.