Probably something so simple
 

But on my site the usernames are quite small, how do i change the font size?

if you'd just like to increase the size of username, use the html markup field in usergroup manager. something like this

I'm assuming you are using a custom style ... can you take a screenshot of what you are referring to? usernames appear in all place - different sizes

for example...
for postbits, you can edit the property/value of the selector 'bigusername' to change how the username looks like (style)

you can find it within STyle Manager / MAIN CSS