It's just a form of jacket or you could use it as a smock, if you made it long enough you could wear it around the house while you're sewing. You can decorate it however you want. You could really make it into anything you want it for according to the fabric and decoration you use.
You decide on a length and width by measuring and adding x amount of inches to the measurement. if you want it down to your waist, hips, knees, or floor; just measure the distance from your shoulder to where you want it and add 12 inches to that measurement. For the bust measurement, loosely measure around your chest and I think you add 6 inches. It's very clear in the diagram, I only glanced at it.
This url gives clear information on how to make the basic garment, if you google it, you'll see different ways people have decorated it.
http://www.thequiltercommunity.com/Wc8f1c1a1f8b9e.htm