CSS: Editing font height in header

Good evening

I'm trying to alter the size of font used in a template I purchased recently - the site is Wordpress based, so I'm not sure whether it functions in exactly the same way. I'm happy with the size of font for all other parts of the site, just the page titles are looking too big. The header code is:

<!-- Header -->
                    <div class="5grid-layout">
                        <div class="row">
                            <div class="12u">
                                <header id="page-header">
                                    <h1><a href="<?php echo home_url( '/' ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home" class="mobileUI-site-name"><?php bloginfo( 'name' ); ?></a></h1>
                                    <?php ax_menu(99); ?>
                                    <br />
                                </header>
                            </div>
                        </div>
                    </div>

I'm trying to edit the size of text that I believe the following code is referring to:

<h1><a href="<?php echo home_url( '/' ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home" class="mobileUI-site-name"><?php bloginfo( 'name' ); ?></a></h1>

I've tried the following code, with no success, and hoped someone could point me in the right direction!

h1.12u {
    font-size: 1.15em;}

Any advice gratefully received.

You've got the h1 and 12u the wrong way around:

.12u h1{
    ...
}

Many thanks for the swift (and patient) reply.

Have made the change, but text size is still huge. I'm not sure if it's a further error on my part (on the code above) or whether it's an issue with my lack of understanding on Wordpress?

Maybe...

#page-header h1{
    ...
}

If you have chrome, look in the dev tools and the elements / styles panel.

[gabrielcastillo]

Try clearing your page cache.