Declare the a element as display: inline-block and drop the width and height from the li element.
Alternatively, apply a float: left to the li element and use display: block on the a element. This is a bit more cross browser compatible, as display: inline-block is not supported in Firefox <= 2 for example.
The first method allows you to have a dynamically centered list if you give the ul element a width of 100% (so that it spans from left to right edge) and then apply text-align: center.
Use line-height to control the text’s Y-position inside the element.