I want to reduce the post header size

First you can install the Custom CSS plugin and add the following code:

nav#mainmenu {
	margin:0;
	padding:0;
	text-align:center;
	position:relative;
	width:100%;
}

This will reduce the space between the navigation and the container.

and

#logo {
	position:relative;
	display:block;
	padding:40px 0 0 0;
	line-height:1.8em;
}

This will reduce the space between the logo and the navigation.

Let me know if this help 🙂

Using the Custom CSS plugin add the code below. Or if you are using a child theme you can modify the code there. See if it looks better.

.pin-article {
	background:#fff;
	margin-bottom:20px;
	border-radius: 2px;
	-moz-border-radius: 2px;
	-webkit-border-radius: 2px;
	box-shadow: 0 1px 3px rgba(34, 25, 25, 0.4);
	-moz-box-shadow: 0 1px 2px rgba(34,25,25,0.4);
	-webkit-box-shadow: 0 1px 3px rgba(34, 25, 25, 0.4);
	word-wrap: break-word
}
.pin-article article.article {
	padding:5px;
}