SIZE SWATCH: we would like to show the sizes available for our configurable products instead of the drop-down. Furthermore, when a size is finished this size should be "crossed"

How can I do this?

Fabrizio, That's a Magento question rather than a theme question 
In product attributes ... choose visual swatch instead of drop down list .. and I think you will find that you will see a line across the swatch if it's out of stock