Beaver Builder Themes

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

.post-article .post-media .media-inner img, .portfolio-image-wrapper img {
    width: 700px;
    height: auto;
}

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/project view, please add this to the Custom CSS area in Customize > Additional CSS:

.post-article .post-media .media-inner img, .portfolio-image-wrapper img {
    margin: auto;
    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 ?