JSFiddle - React, Tailwind, and code Playground

HTML

<select  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 id="sizeBox">test</div>

JavaScript

$(function(){
    $('#sizeSel').on('change',function(){
        var dim = $(this).val();
        var wh= dim.split("x");
        $('#sizeBox').css({'width':wh[0]+'px','height':wh[1]+'px'});
    });
});