If you look at the source code of one of your posts, you should notice the body classes to be something similar to this
<body class="post-template-default single single-post postid-3176 single-format-standard">
so to target all single posts you could use this CSS:
.single-post .main-header{ min-height: 300px !important; }
or to target this specific post, use this CSS:
.postid-3176 .main-header{ min-height: 300px !important; }