Hey,
because both (gallery + blog) are based on the same template it's not possible without using a plugin like: http://wordpress.org/extend/plugins/custom-post-template/
1) You need to create a template:
a) Just make a copy of single.php, rename it to gallery_post.php and paste the following code before the first code line.
<?php
/*
Template Name Posts: Gallery Template
*/
?>
2) In gallery_post.php replace the thumbnail dimensions:
$big_prev_image = kriesi_post_thumb($post->ID, array('size'=> array('L'),
'wh' => $k_option['custom']['imgSize']['L'],
'display_link' => $prev_image_link,
'linkurl' => array ('XL','_preview_big')
));
Replace "L" with any other dimension defined in functions.php or you can even define your own one. Then select the template for gallery entries on your post/page editor page.