Hello @vharel,
There is currently no means provided in the ‘back end’ of the plugin to change the font-size of your popup content. You will have to modify your theme’s ‘style.css’ file to change the font-size of your popup content. We’ve provided users several ways to find the CSS selectors they will need to target.
See: http://docs.wppopupmaker.com/article/147-getting-css-selectors
You can also view and copy the core plugin and popup theme styles from the back end of our plugin for use in your stylesheet.
1) Log into your site, and open the WP Admin.
2) Go to ‘Settings’ >> ‘Assets’ and select the button ‘Show Popup Maker CSS’. Both ‘Core Styles’ and ‘User Themes Styles’ for the plugin and popup themes will display respectively. Paste this CSS directly into your theme stylesheet in order to work with it. You cannot modify the plugin’s CSS styles from this options page.
You will need to target the .pum-content class attribute with a font-size property set to whichever value you want. If you need additional guidance on CSS properties, consult this reference: https://tympanus.net/codrops/css_reference/
I hope this response was helpful.