attributes combined with quantities available??!

Hi, I'm confused! I have a added one product to my zencart.

The product is a vest top that comes in small and medium. I have been able to add the attributes S and M in a drop down box, this is no problem. I have the qty. of the item set at 6 but i don't know how to change the qty. to work per size (or attribute) chosen. For example - i have 6 medium and 3 small available.

Is this doable? I was thinking of instead making new categories (vests/sizes/small and vests/sizes/medium) but this would involve a lot more work and turn the site into a sprawling mass of categories!!

I am totally new to this, it might be quite obvious and i've just missed it! Any help would be much appreciated!!

Stock by attributes is not available in the Zen Cart program at the moment. There is a addon in the downloads sections that may help, but I have never used it myself.