If you currently have this same question you can simply add padding to the right place:
li {
//remove any padding or margin attributes from here
}
li a {
display: block;
padding: 20px; //or however big you want the clickable area to be
}
Anchor tags are by default inline elements, so you have to explicitly change them to display as block elements before you can mess with the padding or the margins.
Hope this helps!