How do you make it bigger, I know you have to use coding because Its not inthe admin CP.

My question is how do u make only the Username font size to make it bigger and more easy to read..

Thnxz in Addvance

That depends on the forum version you are using. Could you provide us with that info please? Check your current version in "Display / Choose a theme / version".

i'm using phpbb3, is it possible then?

It has no own class in phpBB3, but fortunately it's possible with a little trick:

.postprofile dt{font-size:1.5em;}

The red marked value can be modified to adjust the size. Paste this code into "Display / Colors / CSS Stylesheet".

thank you Sunny_D, very helpful