How to change the product title font size at single product page?

Important notice: This product is now in maintenance mode and is no longer being actively developed or supported. Updates and bug fixes will not be provided unless they relate to security concerns.

If you are using WordPress 4.7 or later you could add the code  Admin > Appearance > Customize > Additional CSS. Otherwise we advise you install    Advanced CSS Editor plugin and follow the instructions. In Appearance -> Customize -> Advanced CSS Editor add the following code.

.single-product .product_title{ font-size:20px !important; }