- Click Photo Gallery from the menu on the left of your WordPress dashboard.
- From the list under Photo Gallery, click Options.
- Click the Lightbox tab to access its settings.
- Find Filmstrip Size and set its size in the box. If you can’t find it, please make sure that the option Enable filmstrip is set to Yes.
- Click the Save button at the top right to save your settings. This will apply your selected option to every gallery that uses default options.
In order to apply this setting to a single specific gallery click the shortcode of the published gallery in your editor, uncheck the box next to Use default options, click the Action on image click drop-down menu, follow the step 4, then click Update.