You have not mentioned your URL and which style you are using but here it is (using style2.css as an example). Open the style2.css and locate the below line and change your font % accordingly like so:
h1,.componentheading {font-size:125%;}
Or you can add the following to the bottom of your style2.css using any % to meet your needs
/* HEADINGS CUSTOMIZATION */
h1 {
font-size: 125%;
}
h2 {
font-size: 110%;
}
h3 {
font-size: 100%;
}
h4 {
font-size: 100%;
}
h5 {
font-size: 100%;
}
I think you want to change the Article Title from H1 to H1 but unfortunately, Afterburner template for Joomla 2.5 does not have the default.php file nor the directory "com_content\article".
But here my solution for you:
If you want to control the Headings, like h1, h2 and so on, for SEO purposes, do what I do and it works great:
Go to your Article Manager Options and select 'Hide' option for the 'Show Title'.
Then you enter an H1 using your keyword/key phrase and H2 for the rest of your headings.
What I also recommend is try to use H1 only once per page.
Side note: I tried to setup the mycustomstyle.css file within the Afterburner template to make customization easier and so that any Afterburner upgrade will not affect my customization but the template does not have a folder called 'features' to upload the custom css file to, therefore, I recommend that you backup your style2.css (or the one you are using) just in case.
I hope I understood your question and also hope that the above helps