Want bigger thumbnails? Apply the following CSS to the Style tab of Ari Fancy Lightbox settings:
| Code:  | 
 .fancybox-thumbs { 
  width: 300px !important; 
} 
 
.fancybox-show-thumbs .fancybox-inner { 
  right: 300px !important; 
} 
 
.fancybox-thumbs__list a { 
  height: 150px !important; 
  width: 150px !important; 
} 
 
/* Lets shrink the scroll bar for more room */ 
 
.fancybox-thumbs__list { 
  scrollbar-width: thin; 
  scrollbar-color: #555 #333; 
}
  | 
 
Want even bigger thumbnails? Set the first two widths (300px) to the max container size, then divide your new value by 2 and apply it to .fancybox-thumbs__list a (currently 150px).