dig around in content-extensions.php to see what is generating the thumbnails. you will see that thematic conveniently has a filter called thematic_post_thumb_size that changes the size of the thumbnails.
function full_size(){
if (is_tag() || is_category()){
return "full"; // also accepts an array of height/width array(100,100)
}
}
add_filter( 'thematic_post_thumb_size' , 'full_size' );
you can totally change the conditional logic to suit your purposes. is_archive() would work for date,author, tag, category and basically ALL archives. or you could do away w/ the conditional logic completely to have the full image anywhere the excerpt is displayed...
ps- i really like your background, but your site is hard to read b/c of the low contrast between the gray bg and the gray text... just imo