1. If you wish to hide the featured image of a project post in single view
Please add the code below to Customize > Additional CSS
Here is a guide how to add custom css: How to add custom CSS / PHP code or modify the theme code
.post-article .post-media .media-inner img, .portfolio-image-wrapper img {
display: none;
} 
2. Or you can simply decrease the Featured image size in a single project view:
Please add this code in the Additional CSS field:
.post-article .post-media .media-inner img, .portfolio-image-wrapper img {
width: 800px;
}