component.js

class MyComponent extends React.Component {
...
render() {
    return (
        <div className="parent">
            <div className="child" ref={el => this.targetEl = el}>Content Here</div>
        </div>
    )
}
...
)

target.js

const dropTarget = {
  hover: (props, monitor, component) {
    // Get the target bounding rectangle
    const hoverBoundingRect = component.decoratedComponentInstance.contentEl.getBoundingClientRect();
    // ... 
  }
}