How to implement ads JS files for different screen sizes?

Hi,
Sorry for any inconvenience in advance!
I can’t make sure this question whether it can be solved from you. Or it is out of range from you can handle.
I am still investigating this issue now. So far, I can’t find out any clue as this subject. So, could you please helpt to give any advice if available?
I tried to add below codes into [Text Widget] to reach this request. But it seems can’t work properly.

<script type="text/javascript">
function big_size() {
var element = document.createElement("script");
element.src = "/big_size.js";
document.body.appendChild(element);
}
function small_size() {
var element = document.createElement("script");
element.src = "/small_size.js";
document.body.appendChild(element);
}
if(screen.width < 768)
{
 window.onload = small_size;
} else {
   window.onload = big_size;
}
</script>

Thanks for a million!