How to use the thumbnail size with catch_that_image

I can’t answer your question directly but I will give you a much simpler and more powerful solution to accomplish the same thing.

Create a Custom field in the posts which should display thumbnails in a loop somewhere.
Call it thumb or whatever you like. Upload a thumbnail image and paste its full url in the Custom field value field.

Now put the following in your loop where you want the thumbnail image to show up:

<?php $image = get_post_meta($post->ID, 'thumb', true); if($image) : ?>

Set width in css styling your loop to control the size of the thumbnail image. Preferably, create small resized thumbnail images in a photo editor and upload them. It will fasten pageload and save you bandwidth.

The method also lets you display a thumbnail image of choice. You can also have posts without images while they display thumbnails on your frontpage or on other pages.