Full Size Image in WordPress Default Gallery
WordPress Gallery is indeed a handy tools to neatly insert your collection of pictures and order them easily in your post, without being bothered by performing manual arrangement (such as using tables, etc.) through the HTML editor. But there was this time when I feel annoyed because when one of the image in the gallery was clicked, the loaded page didn’t show a full scaled image, instead it displayed a medium-size.
I then realized that this is something that related to theme’s functionality, because there is no option to set the image size in the gallery setting.
The solution is by adding a specific template that handles the image attachment page. If you check your wp-content/themes folder, you will notice that those files that are found in there, are templates which WordPress uses to display the content of your blog (or the category, or the attachment).
By default, WordPress uses the single.php file as the template that handles all your image attachments. And this default is something that we should tinker with 
