How do I make an <a> tag the size of it's parent <li> tag for larger clickable region?

As others have said

li a { display: block; }

should achieve what you're after. But you must also remove any padding from the <li> and set it on the <a> instead. For example:

li { padding: 0; }
  li a { display: block; padding: 1em; }

In CSS:

li a {
     display: block;
}

Of course, you'll want to make your selector more specific than that.

<ul>
    <li class="myClass">
        <a href="#">Link</a>
    </li>
</ul>

li.myClass a {
    display: block;
    background-color: #fdd; /* Demo only */
}

http://jsfiddle.net/userdude/jmj2k/