Welcome to the world of Quickbooks Inventory
To my knowledge, only their Point of Sale software has the capability of handling size/color/design issues. If you are using their lower level (like I am), you would need to create a separate product for every combination of garment, design, color and size.
Do you also track how many blanks you have? They would need to be separate products as well.
What I ended up doing was creating the blanks and inventory products and the designs/colors as services. As I create finished garments, I create an inventory assembly that is a combination of the design/garment/size/color/design/color. That way I can tell by design what I have in stock and also by blank garment what I have in stock.
It's a royal PITA but I haven't found a better way...