Hi -
Here's the short answer - you have to modify the css of your coraline child theme.
Go to your Dashboard. Go to Appearance --> Editor.
Select Coraline Child Theme - style.css.
The CSS of the child theme overrides the CSS of the parent Coraline theme.
Paste in some code at the bottom of the Coraline Child Theme style.css, replacing the height I've put in with the dimensions you want (you probably want to keep the width the same):
#branding img {
width:990px;
height:400px;
}
Click on Update File.
Long answer:
If you know CSS, and if you're on a PC get Firefox and the Web Developer plug-in. That lets you pinpoint what CSS to change to change the look of things. Then Google for a tutorial on how to use it. (It's a bit of a cumbersome process but it works.)
I don't know what to suggest if you're on a Mac.
There's a lot to learn. But it's all do-able.
Here's a link to the WordPress Codex article on child themes:
http://codex.wordpress.org/Child_Themes
Good luck!