expanded.js

var iframe = document.querySelector('#myid')

window.addEventListener('message', function (ev) {
  var message = ev.data.split(':')
  if (message[0] === iframe.id) {
    iframe.style.height = message[1] + 'px'
    iframe.style.width = message[2] + 'px'
  }
})

iframe.contentWindow.postMessage('yo:' + iframe.id, '*')