If you want to customize the Page Hero section – the large section at the top of your pages which contains your post title – this article will help!
Customizing the Height of the Hero Section
You can try using this custom CSS (add this in Theme Options > Custom Scripts > CSS). Please note that it will affect all pages:
.op-blog-header-content{
min-height:300px;
}
Change 300px to the height you require.