Change caption text size

Hi,
Apologies for the delayed response.

You can use following CSS code:

body .dgwt-jg-gallery .dgwt-jg-caption span {
font-size: 15px;
letter-spacing: 2px;
}

How to add custom CSS? Take a look here: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-css-to-your-wordpress-site/