how do you increase the height of an html textbox

Question

I’m assuming from the way you worded the question that you want to change the size after the page has rendered?

In Javascript, you can manipulate DOM CSS properties, for example:

document.getElementById('textboxid').style.height="200px";
document.getElementById('textboxid').style.fontSize="14pt";

If you simply want to specify the height and font size, use CSS or style attributes, e.g.

//in your CSS file or <style> tag
#textboxid
{
    height:200px;
    font-size:14pt;
}

<!--in your HTML-->
<input id="textboxid" ...>

Or

<input style="height:200px;font-size:14pt;" .....>