what is the memory size of a weakmap object?

Question

For example,
Suppose I have a weak map which links some dom element with another dom element like this:

let item1=document.querySelector("#item1");
let item2=document.querySelector("#item2");

const wMap = new WeakMap();
wMap.set(item1,item2);

This solution returns the byte value of the object
similarly, what would be the size of the weakmap in this case?