Hello, Wade Yang,
Thank you for the reply.
Do you mean product title on a single product page?
There is no option to change the font display size on the mobile phone only, but you can do this using custom CSS code. For example, add the next code in Theme Options > Theme custom CSS > Mobile:
.single-product .product_title {
font-size: 30px !important;
}
If you have any additional questions or concerns, please do not hesitate to reach out to us. Our support team is always available to assist you.
Kind Regards,
8theme team