[box2D] how positioning and scaling work

Hello,
i start looking for some Box2D examples today and everything is working fine except that i don’t understand the creation of a simple rectangle inside Box2D (polygon), if am right the steps needed to create a box2D rectangle are the followings


       

		BodyDef polygonDef = new BodyDef();
		polygonDef.position.set(x, y);
		polygonDef.type = BodyType.DynamicBody;

		Body polygonBody = world.createBody(polygonDef);

		PolygonShape polygonShape = new PolygonShape();
		polygonShape.setAsBox(width, height);

		fixture.shape = polygonShape;
		polygonBody.createFixture(fixture);

		

and to create a Rectangle in libGdx we do this :



		shapeRenderer.begin(ShapeType.FilledRectangle);
		shapeRenderer.filledRect(x, y, width, height);
		shapeRenderer.end();

now supposing i create a libGdx rectangle with the following coordinates :
x =100, y=100, width=100, height=100
in order to have a Box2D rectangle in the same position with the same size, i had to use the following coordinates :
x = 30, y=30,width=10,height=10;

i tried to figure out the relation between them but i didn’t succeed, i know that box2D use meters instead of pixels but what i don’t know is 1meter=??pixels, and how i can use Box2D with “simple” coordination

thank you