Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

body.single-product .woocommerce-price-suffix {
    font-size:20px;
}

If you want to change the font family, You can change the font by means of the Advanced typography option, which allows you to choose the item from a drop-down or insert your custom CSS class. Please navigate to the Theme Settings > Typography > Advanced Typography.

Here is the documentation providing more detailed instructions: https://xtemos.com/docs/woodmart/advanced-typography-settings/

You can use this custom selector: body.single-product .woocommerce-price-suffix

Best Regards