How to increase font-size of the su_box title

Checking the source code generated you find that then su_box title element is getting a style like this by default:

.su-box-title {
display: block;
padding: 0.5em 1em;
font-weight: bold;
font-size: 1.1em;
}

Try to overwrite the settings (font-size) in custom css for your WP/theme. I found, though, that the output still uses the default style settings, regardless of what I write into my custom css (using Plugin My Custom CSS). It may work for you, though.

Yes, I had to use that custom css, as I did not see any shortcode parameter for the su_box to control the font size of the title.
If I missed anything please let me know and I will get rid of the custom css.