1. In order to decrease the featured image size in a single post view, please add this to the Custom CSS area in Customizer > Additional CSS:


.post-article .post-media .media-inner > img {
    width: 700px;
}



You can edit the width number as you prefer.


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2. In order to decrease the featured image size in a single portfolio view, please add this to the Custom CSS area in Customizer > Additional CSS:


article.type-jetpack-portfolio .portfolio-image-wrapper > img {
    width: 700px;
    height: auto;
}



You can adjust the width number as you prefer. This code will prevent the image to appear in its original size.


Here's a guide how to add the code: 

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