Authors Widget padding

you can edit your stylesheet to add padding. This here explains how to add margin, padding should be not difficult to do as well:

http://blog.fleischer.hu/wordpress/authors/comment-page-1/#comment-28456

I guess that another stylesheet overrides this. Try adding some id-s as well:

#sidebar .widget_authors li {
padding:0 0 10px 0;
}

or

#sidebar ul li.widget_authors ul li {
padding:0 0 10px 0;
}