A web-shop presents a product. The user wants to select a size. The size can be presented in some way (dropdown list, all visible at once, etc... not important).
The product is out of stock, but that doesn't mean it will still be so one week later. That's why the user has the option to be notified once his size is available again.
What is the best way to make the user understand that he has this option, instead of him just leaving the site because he couldn't get his desired product?
A little demonstration so we have the same mental image:
Adidas, for example, doesn't show the unavailable sizes and intead just displays the question "Is your size out of stock?". On click it presents a dialog where you can enter your email (the example is from the german site).
I've seen others only pesent this option after you click on one of the not available sizes.
What do you think?