In order to decrease the featured image size in a single post view, please add this to the Custom CSS area in Vamtam >> General:

.post-article .post-media .media-inner > img {

display: inline-block;

width: auto;


Here's a guide how to add the code: 

How to add custom CSS / PHP code or modify the theme code ?

After this change if you unload an image with 400/400 px size it will appear in the post 400/400 px - its original size.