How do I change the fontsize for usernames on the post bit?

[MGP_Tech]

Under the StyleVars for your template is the Common section and in there is Font. If changing that doesn't work then find the postbit.css template and look for (I'm using legacy style postbit)

.postbitlegacy .userinfo a.username, .eventbit .userinfo a.username {
clear:{vb:stylevar right};
font-size:{vb:stylevar font.fontSize}{vb:stylevar font.units};
font-weight:bold;
width: auto;
max-width:{vb:math {vb:stylevar postbitlegacy_userinfo_width} - {vb:math {vb:stylevar padding}*2} - 20px};
word-wrap:break-word;
}

and you can change the font size manually there.