There are two places you can do this.
* Set the SCLogin module > Social Button Settings > "Image Style" setting for each of the social network providers. If you'd like to just change the SCLogin module button sizes, you can do it here.
* If you'd like to change it for any time the login buttons are displayed (e.g. the Joomla login page, with easy-tags, etc), also set the JFBConnect backend > Configuration > "Default Login Button" setting for each provider tab.
-Melissa