How to change the thumbnail image size
Step 1
In your WordPress administration area, go to Sunshine > Settings > Galleries
Step 2
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.
Step 3
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.
Step 4
Go to “Tools > Regen. Thumbnails” and initiate the thumbnail regeneration.