h2.woocommerce-loop-product__title {
    font-size: 20px;
}

To add CSS:

• If you are using WordPress 4.7 or later, use the “Additional CSS” option in the customizer.
• If your theme has a custom CSS option, use that to add the CSS shown above.
• If not, install the plugin Simple Custom CSS.

Learn to use the Chrome Developer Tools to help you see and test changes to your CSS.

https://i.imgur.com/VcwwHH1.png

You could target the post ID as well for this specific product.

li.post-17828 h2.woocommerce-loop-product__title {
    font-size: 20px !important;
}

Best regards,
Konstantinos

You can simply add a Plugin like @sterndata pointed out and make it easier on yourself without accidentally messing something up.

Best regards,
Konstantinos

Best regards,
Konstantinos