It is very easy to change the wordpress default thumbnail size at any time. Login to your wordpress and go to Settings->media and use the right options or change size of thumbnail with the crop option unchecked.
Then use this plugin to regenerate the thumbnails
http://wordpress.org/extend/plugins/regenerate-thumbnails/
Or if it still doesn’t work, that means your theme is overriding some settings.
I would recommend creating a different size manually and then use it in your templates. Its easy. Just add the following code to your functions.php and it will allow you manual control over new size and will add it to the image size list in media upload box (the pop up shadowbox)
add_action('admin_head', 'wphd_hide_dashboard', 0);
if ( function_exists( 'add_image_size' ) ) {
add_image_size( 'custom-thumb', 300, 9999,false ); //300 pixels wide (and unlimited height, 9999 is large number so as to neglect the height ), cropping set to false
}