All image sizes in every CSSIgniter theme are declared in the file functions.php, in the main folder of your theme.
Opening that file, find the following lines of code (or lines that look like this):
// // Define our various image sizes. // add_theme_support( 'post-thumbnails' ); set_post_thumbnail_size(640, 300, true); add_image_size('featured_blog', 690, 220, true); add_image_size('gallery_thumb', 314, 314, true); add_image_size('slider', 1140, 585, true); add_image_size('gallery_full', 1140, 450, true); add_image_size('carousel', 180, 94, true); add_image_size('wide_thumb', 270, 125, true);
The above code is taken from the functions.php of a random theme of ours, SixtyOne (from our website).
In every line:
add_image_size('slider', 1140, 585, true);
The text in quotes is the name of the position of the thumbnail, and usually describes where the thumbnail is used.
So for the above code, the 'slider' thumbnail is used on the fullwidth sliders in SixtyOne's frontpage and the 'gallery_full' is used in SixtyOne's full width room gallery templates.
If you have any doubt on if where a size is used you can always right click on the image, select "Inspect Element" in Chrome or Firefox and check out the class of the <img> tag as shown in this screenshot: http://d.pr/i/9xJC.
It will be named attachment-name_of_thumbnail (as declared in functions.php).
The first number of the declaration is the width of your image, and the second one is the height. Change those numbers, and then install and run a plugin such as Regenerate Thumbnails (http://wordpress.org/extend/plugins/regenerate-thumbnails/) in order for your images to be recropped to the new dimensions.