How to change the thumbnail image size
In your WordPress administration area, go to Sunshine > Settings > Galleries
Enter new dimensions for your thumbnail images. Check the “Crop” option if you want the images cropped to the exact dimensions entered, or leave unchecked to use those dimensions as a maximum height/width but maintain aspect ratio.
Changing these settings will not automatically update all the thumbnail images in your galleries. This currently requires a separate WordPress plugin, Regenerate Thumbnails. Install this plugin and activate it.
Go to “Tools > Regen. Thumbnails” and initiate the thumbnail regeneration.