Thumbnails help

Hello,
I use this code to show different thumbanail size in my cotegories.

<?php
if ( has_post_thumbnail() ) {
if (is_category('54')){
the_post_thumbnail( 'category-thumb' );
}
}
?>

How can i change that code, when user is in other category, he see normal size thumbanils..

this is in functions.php

if ( function_exists( 'add_image_size' ) ) {
	add_image_size( 'category-thumb', 300, 300 );
}

Here you can see my problem… now i see small and big thumbnails in this category (54).

link : http://stefanvblog.com/category/interview/