According to the style guide provided by A Tailored Suit, one way to judge for proper length is to relax your arms straight at your sides. Cuff your hand around the hem of the coat. The hemline should just touch the ball of the fingers.
Measure from the top of the spine to the floor and then divide the resulting number in half. That is the proper length for a men's jacket, according to A Tailored Suit.
While neither method is foolproof, measuring will give you the best result. While there is a correlation between the length of a man's arm and his height, if your arms are either longer or shorter than average, the cuffed hand technique may not work as well for you.