Measured to not hang over your hands or rise up your arms, the sleeves should land somewhere between your wrist and your thumb. When buttoned, the lapels of your blazer should only be loose enough to fit a tightened fist between the fabric and your chest, and they should be snug to your chest.
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.
