RE: Thumbnails linked to post
An example of that resides on my Post Image page, but in short you just stick post_image() in a link tag, using WordPress’ the_permalink() template tag to output the permalink:
<a href="<?php the_permalink(); ?>"><img src="<?php post_image('/path/to/default.gif', TRUE, FALSE); ?>" /></a>
The second (use_thumb) and third (img_tag) attributes set Post Image to use thumbnails, and to only echo the url to the image.
RE: Change thumbnail size
The plugin can’t do anything about the actual thumbnail file, but if you need thumbnails to be displayed in the same dimensions each time, you could add width and height attributes to your img tag:
<a href="<?php the_permalink(); ?>"><img src="<?php post_image('/path/to/default.gif', TRUE, FALSE); ?>" width="200" height="150" /></a>