I'm assuming you're using the {JFBCLogin} button? For maximum flexibility and styling options, you may want to use a custom image for the Login with Facebook button. With that, you can create whatever style and size and wrap it in whatever class you want. For a custom image, simply use the following code:
<a href="javascript:void(0)" onclick="jfbc.login.login_custom();"><img src="/linktoimage.jpg" /></a>
You can see more information about the custom login buttons in our
Joomla Facebook Support questions
article.
Hope that helps, but if not, let us know,
Alex