That one isn't actually in the css file.  That one is located within the "single.php" file. 

Search for this code in single.php

<div class="mi"> <?php if($pli){ ?><img src="<?php bloginfo('template_directory'); ?>/phpthumb/phpThumb.php?src=<?php echo $pli; ?>&w=350&h=350&iar=1" alt="<?php the_title(); ?>" /><?php } else { ?><img src="<?php bloginfo('template_directory'); ?>/phpthumb/phpThumb.php?src=<?php bloginfo('template_directory'); ?>/imgs/no-image-available.gif&w=350&h=350&iar=1" alt="<?php the_title(); ?>" /><?php } ?> </div>

Within this code, are the height and width attributes for the images. (w=350&h=350)  You'll want to change both instances.

Check out this post paying particular attention to post #4:

http://www.markettheme.com/forum/support-area/main-image-dimensions-ratio/