Display medium size image instead of thumbnail

Solutions anyone?

OK. Let me put it another way –
I was using the catlist plugin to display a list of related posts below the content. My requirement then was of small images (150 x 100 px) but now I require to show bigger images (400 x 300 px).
Everything works fine but the images are blurred. I understand this is because I am using the thumbnail size images. How do I use medium size images instead of thumbnail?
I guess I have to change something in this template file code –

$lcp_display_output .= $this->get_thumbnail($single);

No solutions so far.
Guess I have to start hunting for a new plugin ??

Line 437 in lcp-catlistdisplayer.php (assume you are using your own template.php):

$info = $this->catlist->get_thumbnail($single, $thumbnail_size = ‘large’, $lcp_thumb_class);