<xen:avatar user="$visitor" size="m" img="true" />
sidebar.css along with style property visitorPanelAvatar.
The avatar HTML image tag contains width and height attributes so there is no need to specify these in CSS and the CSS actually over-rides the html attributes.
It just seems strange to me that you cant change the size to "s" and not have it take effect. It seems weird also that sizes need to be specified in both css and html attributes on the image tag.