HOW TO MEASURE
Chest — Measure around the fullest part of your chest, under the arms. Your body chest in inches is your suit jacket size: a 42" chest takes a 42.
Shoulder — Measure across the back from one shoulder point to the other. The seam should sit on the edge of your shoulder, not past it.
Sleeve — Measure from the shoulder seam to the cuff. A jacket sleeve should end about half an inch above your shirt cuff.
Back length — Measure from the base of the collar seam straight down to the hem.