Size of 'small image'

When I view my site and select a subcategory I am shown small images for each item in that subcategory, and also in the panel that displays each item as a special for that month.
Exactly how can I control/define the particular category image and especially it's displayed image size?
I need specifics as everything that I try does not change the small images displayed size, and I am suspecting that my installation is abnormal.
But what is normal behaviour?
Or should I describe exactly the site behaviour that I want to achieve, and nearly have, and find out that perhaps it is unnatainable?
OK I will.
I have two images of the same object, one a medium size and one a large size. They are in the 'medium' and 'large' subdirectories of the main 'image' directory. Named foo_MED.jpg and foo_LRG.jpg respectively.
I want the medium size image to display everywhere that a small image is normally displayed, and the large image to be displayed on the Product Page with the Product Enlarge facility disabled.
I have the Product Page working as I have described, but am unable to get the medium size image displayed at the desired size.

KiwiBrian:

...I have two images of the same object, one a medium size and one a large size. They are in the 'medium' and 'large' subdirectories of the main 'image' directory. Named foo_MED.jpg and foo_LRG.jpg respectively.
I want the medium size image to display everywhere that a small image is normally displayed, and the large image to be displayed on the Product Page with the Product Enlarge facility disabled.
I have the Product Page working as I have described, but am unable to get the medium size image displayed at the desired size.

A silly suggestion (which is all I can come up with)...
Make a copy of the MED image and put it in the folder that would ordinarily hold the small images. If that still doesn't work, I believe there is a setting in admin somewhere that controls the size of the images - probably under configuration/images or near that. Make sure it is set so that a small image space will hold the MED image.

HTH

Rob

Thanks for reply.
Regardless of what I try, I am unable to change the displayed size (100 x 60) of the small image that is displayed in the items contained within a selected subcatalog menu entry.
Anybody that can describe if I should be able to, and if so - how?