The Boundary link tool renders a rectangular border to show the bounding box of the link. It accepts a few additional arguments, which can be passed as an object to the link tool constructor:

padding number|object This option determines whether the boundary area should be visually inflated and if so, by how much. Default is 10 ({ left: 10, top: 10, right: 10, bottom: 10 }).
useModelGeometry boolean If this option is set to true, the position of the boundary is calculated based on the dimensions of the link model.

Example:

var boundaryTool = new joint.linkTools.Boundary({
    focusOpacity: 0.5,
    padding: 20,
    useModelGeometry: true
});