I patched some code to solve this, and allow the use of thumbnail, medium, large or original image size.
At Line 2363
$arr = wp_get_attachment_image_src(get_post_thumbnail_id($page->ID), 'thumbnail');
$str = str_replace("[post_thumb]",$arr, $str);
$str = preg_replace_callback('#\[ *post_thumb *(size=[\'|\"]([^\'\"]*)[\'|\"])? *\]#',
Add the callback function:
$arr = wp_get_attachment_image_src(get_post_thumbnail_id($this->page->ID),$matches);
Then, the shortcode for [post_thumb] can be used as [post_thumb size="thumbnail"]
medium and large values can also be specified, if size is empty or size parameter is not present original file will be returned.
Thanks a lot for your useful plugin !