The simplest thing you may want to try is to update the language string you've already modified to add the proper h-tag. So, instead of:
COM_JFBCONNECT_REGISTER_WITH="Register With "
Try:
COM_JFBCONNECT_REGISTER_WITH="<h3>Register With These Social Networks</h3>"
(Or whatever you want). The h-tag will give you the size you want and add a line break after automatically.
Beyond that, all the login buttons are put into a container class of "sourcecoast login", which you can target in your CSS however necessary. Each login button also gets it's own class of "social-login facebook" (or whatever social network the button is from.
I hope that helps give you a starting point, but if you need anything further, just let us know.
Thanks,
Alex