That's correct, the font sizes can be changed through the custom style editor.
The problem of the custom style editor is that it only lets you customize the 'light' style design of the slideshow. This is something that will be improved soon.
In the mean time you could look up the 'dark' stylesheet of the slideshow by going into your WordPress installation and looking for this folder: '
wp-content/plugins/slideshow-jquery-image-gallery/style/SlideshowPlugin/'. In this folder you'll see the file 'style-dark.css'. Open it with any text editor and copy its contents. After copying, return to the slideshow and paste the copied stylesheet into the custom style editor. You can now adjust the font size settings.