Dimensioning and positioning is set by bounds (Rectangle) or by center and radius (Circle). Circle radius is given in meters. Those can be set as options or by special methods setBounds(), setCenter() and setRadius(). They are documented already.
getBounds() for Circle would be a useful method that is missing.
Altogether those are very handy classes and we can forget my boundsbox.js library in most cases. Image overlaying is still a perfect task for boundsbox.
Quickly testing Rectangle().