Lightgallery Lightbox Thumbnails – how to change single thumbnail size

Is seems that thumbnails at the bottom of Lightgallery Lightbox correctly show the smallest size of picturers (thumbnail size). However we have them too small, and it makes them look bad i.e. blurred as they are scaled up.

We can make changes on our own, but could you please point us to the right direction. How, what files shall we edit so the change can be made to the image size for thumbnails used in Lightgallery Lightbox in order to show size ‘medium’ rather that ‘thumbnail’.