Hello!
You can do this with the Product Variants module:
1. If you have many products having the same "sizes" attribute, edit the first product in the set create a new product class for it. For the rest of the products just select the same product class.
http://drive.google.com/uc?export=view&id=0B30841nNJgxCMzc3OHNrZFRtaGs
2. Now add your attributes in the section specific to your product class ("Global" subtab). Ticking the "multi value" checkbox will let you to specify multiple values for the attribute.
http://drive.google.com/uc?export=view&id=0B30841nNJgxCcDZnYWxDcWQ2QmM
3. Now go to "Variants" tab and choose attributes which variants are to be created from.
http://drive.google.com/uc?export=view&id=0B30841nNJgxCaURXd3NRcERWazQ
4. Now you can specify SKU for each variant in the list and disable variants which should not be available for sale.
http://drive.google.com/uc?export=view&id=0B30841nNJgxCVU1mV1ROc2JyYnM