Generate multiple thumbnail sizes

Great product, but I'm looking for a way to have the system generate more than just the maximum size image and thumbnail image. I would like to have it create an intermediate size image. For example:

Currently full size image is in:
/userfiles/images
And thumbnail is in:
/userfiles/_thumbs/Images/

I would like to have a third size created, something like:
/userfiles/_medium/Images/
and I would like to set the max width and height for this image

I see the following three lines in the config:

$config['plugin_imageresize']['smallThumb'] = '90x90';
$config['plugin_imageresize']['mediumThumb'] = '120x120';
$config['plugin_imageresize']['largeThumb'] = '180x180';

But I can't see how to utilize this.

Does this functionality already exist and I am just missing it? Appreciate the help in advance