How do I make the text box bigger in HTML/CSS?

This will do it:

#signin input {
 background-color:#FFF;
 min-height:200px;

}

According to this answer, here is what it says:

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;" .....>

Try this:

#signin input {
    background-color:#FFF;
    height: 1.5em;
    /* or */
    line-height: 1.5em;
}

If you want to make them a lot bigger, like for multiple lines of input, you may want to use a textarea tag instead of the input tag. This allows you to put in number of rows and columns you want on your textarea without messing with css (e.g. <textarea rows="2" cols="25"></textarea>).

Text areas are resizable by default. If you want to disable that, just use the resize css rule:

#signin textarea {
    resize: none;
}

A simple solution to your question about default text that disappears when the user clicks is to use the placeholder attribute. This will work for <input> tags as well.

<textarea rows="2" cols="25" placeholder="This is the default text"></textarea>

This text will disappear when the user enters information rather than when they click, but that is common functionality for this kind of thing.