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.