A suit jacket size is determined by a person's height and chest measurements. The chest measurement is taken by measuring the circumference of the chest at the widest point in inches.
The combination of these two measurements provides the size number and determines whether a person needs a short, regular or tall size. Men over 6 feet tall typically need to look at tall sizes for a suit jacket. Local tailors can measure a man for his exact jacket size. Some retail locations that sell men's suits have employees who are trained to measure men to determine their suit jacket sizes.