How to get mobile font styles using custom code block

Has anyone figured out how to add styles to a custom code block for mobile font styles? I want some text in my emails to display a few pt sizes smaller on mobile, and I've been troubleshooting every method I can think of with no success.

I've been able to add a class to center align all paragraph text on mobile with the code snippet below, but I can't recreate this effect with font sizes. 

<style>

@media screen and (max-width: 480px) {
p {
text-align: center !important;

</style>