Track inventory on options

I am selling t-shirts and I want to keep inventory of each design and how many of each size I have of each design. How would I go about doing this? I already have a global option for t-shirt size, but how do I specify how many of each size I have in stock?

Also, if a certain size is out of stock, is there a way for them to still place the order for that size so it will in essence be a backordered item.