Box2D uses MKS (meters, kilograms, and seconds) units and radians for angles.
Box2d uses the unit meter to measure its world and the objects within. Things should be between 0.1 - 10 meter for a proper simulation. When drawing on the screen , a certain scale is needed to convert from the meter to pixels. Over here I have used a scale of 100, which means, an item 1 meter wide would be drawn as 100 pixels wide.