Q: What is the dimension of the image that would be suitable for a Header Banner? 

A: The dimesion of the header banner should be at least 540px by 1900px height and width respectively.

Q: How can I reduce the height of the banner area, to make more content visible?

A: If you want to change the height of the banner container go to your FTP server and then find the header.php file of the theme usually found in this path

 concrete5_root/packages/jb_black_knight/themes/jb_black_knight/elements/header.php 

At line 32 change the value where it says 647px to any number you want.

 <?php if ($c->isEditMode()) { echo "height:400px;"; } else { echo "height: 647px;"; } ?> 

Q: How do I modify the spacing on the top and the bottom of the banner?

A: The spacing above the banner can be modified in the same file at line 31. You should find this line:

 echo($c->getAttribute('header_banner')->getVersion()->getRelativePath()); } ?>') no-repeat fixed 51% 155px; 

Change the 155px to any value you want, the smaller the value the thinner the spacing it will be above.

The bottom spacing can be modified via this file:

 concrete5_root/packages/jb_black_knight/themes/jb_black_knight/style.css 

At line 146 you'll see this code:

 padding: 78px 0; 

Again change the 78px to any value you want, the smaller the value the thinner the spacing it will be below. Take note however, this will also affect the spacing in between the sections.