Get bounding box size of compound nodes

[celine3d]

Hi I am using cytoscape with compound nodes with the usage of cytoscape-extend-collapse. On before expand I would like to know what is the bounding box of children, I checked with autoWidth and height but they seem to be undefined. Is there a way to tell? My piece of code:
cy={cy => {
setCy(cy);
cy.nodes().on('expandcollapse.beforeexpand', e => {
const targetPosition = e.target.position();
const targetBoundingBox = ???;
});
}}