JSFiddle - React, Tailwind, and code Playground

HTML

<select onchange="Change()" id="sizeSel">
<option selected>Please Select</option>
<option>HP to decide</option>
<option value="45x40">45x40</option>
<option value="50x20">50x20</option>
<option value="50x30">50x30</option>
<option value="63x30">63x30</option>
<option value="75x40">75x40</option>
</select>
<div style="border:1px solid red;" id="sizeBox"></div>

JavaScript

function Change()
{
    var boxsize = $('#sizeSel').val();
    var strs = boxsize.split("x");
    
    $('#sizeBox').width(strs[0]);
    $('#sizeBox').height(strs[1]);
}