You’ll have to choose which way to go with representing the sizes. (and keep in mind that they are working on changes to configurable products)
The first thing you should decide is whether you are tracking inventory or not.
If not, you can use empty simple products as your sizes and associate them with various configurable products that contain all the product information. The sizes are reusable. This is basically like defining a variable attribute (which does not exist).
If you are tracking inventory you could use custom options, or make each product you want to track a simple product and then associate them with configurable products in a way that makes sense. (I don’t like this method, but I’m not tracking inventory either.)