JSFiddle - React, Tailwind, and code Playground

HTML

<div id="one">
<div class="options-parameters-input">
This is testing
</div>
</div>
<br /><br /><br /><br /><br /><br /><br />
<table width="750" border="1" cellspacing="3" style="float:left;">
  <tr>
    <td>Type font size</td>
    <td><textarea id="heretype"></textarea></td>
  </tr>
</table>

CSS

#one{
    height:100px;
    width:100px;
    background-color:#CCCCCC;
    border:none;
    margin:10px;
    float:left;
}

JavaScript

$("#heretype").on("keyup",both);

function both(){
$(".options-parameters-input").css("fontSize", this.value + "px");
}