It is easy to Adjust The Thumbnails Navigation Box Size of your product gallery with the Woo Gallery Slider Pro plugin. Follow the steps below:

(1) From your WordPress Dashboard Menu, navigate to WooGallery Slider > Gallery. 

(2) Under the Gallery settings tab, you will find the Thumbnails Navigation. Enable the option. 

(3) After enabling this, you will see the Thumbnails Navigation Box Size option in the bottom area with various customization options. Set the Height and Width values as you want in (pixels) or (%) or (em)

(4) Now click Save Settings from the top-right corner and refresh the product gallery to see the changes.