<div ref="parentDiv" class="parent_div">
   <div class="child_div"></div>
   <vue-component
      ref="vueComponentSize"
   />
</div>

<script>
export default {
   mounted() {
      console.log(this.$refs.parentDiv.getBoundingClientRect().height)
      console.log(this.$refs.vueComponentSize.getBoundingClientRect().height)
   }
}

When I try to console.log() the value of ref in mounted it doesn't work either and I am not able to get any kind of data from vueComponentSize ref.