To change the default size of the header image, you’ll need to edit the functions.php file in the theme. Go to Appearance → Editor and then select the file named, “functions.php” from the list of theme files on the right-hand side.

Scroll down until you see these two lines (lines 122 and 123):

define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyeleven_header_image_width', 1000 ) );
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyeleven_header_image_height', 288 ) );

You can manually change the dimensions (1000 and 288), and then re-save this file.

BUT, if you ever upgrade this theme, you’ll lose your changes. So, it’s typically a better idea to create and use a “child theme” to make changes like this. And here is a tutorial that outlines exactly how to do that:

http://www.blog.web6.org/twenty-eleven-header-image-how-to-change-size/

Hope this helps!
Shawn

Open that file in the editor, and then look for this PHP instruction on line 73:

<?php bloginfo( 'name' ); ?>

This line of code tells WordPress to insert the blog name, which you set in Settings > General. I would recommend leaving this alone, and changing your site name (title) in Settings > General instead.

To align it in the center of the page, you’ll need to edit the theme file named, “style.css.” Lines 511-514 control the position of the site title.

Replace, “margin-right: 270px;” with, “margin-right: 0;” and then add a new line, immediately underneath this line, with this instruction: “text-align: center;”

Now your title will be in the center of the header.

The subtitle is automatically generated by WordPress, using the Tagline you specified in Settings > General. This tagline is inserted into the header by this PHP instruction on line 66 of the file, “header.php”.

<?php bloginfo( 'description' ); ?>

You can simply replace this line of code with your preferred text, if you like. But it might be better to simply change your site’s description in Settings > General. This line already appears on the left-hand side of the page.

To add your third line, you’ll need to create a new H2 line, similar to the one you just moved. The easiest way to do this is to copy and paste the H2 containing the site description, and then give this new H2 its own, unique ID. Here’s an example:

<h2 id="my-tagline">Your 4-word statement here.</h2>

Save your changes, and then return to the theme editor, opening the file named, “style.css.” Lines 528-533 control the style for the site description.

Add a new line with this instruction: “float: left;”

Then, add this new set of instructions just below line 533:

#my-tagline {
float: right;
clear: none;
}

You may have to experiment with the positioning, especially since this will place your right-aligned tagline directly underneath the search form. But this should at least get you started.

It sounds harder than it is, but this is actually a fairly simple modification. So, welcome to WordPress theme editing!

All the best,
Shawn