I can think of a way that you could do this but it is fairly custom and not a supported feature of the cart. The idea is that you would have static add to cart buttons for each available item sizes on this page, each item with a different size would be considered unique.
Once you have got all of the add to cart server behaviors on the page the next thing to do is to combine all of the forms so that there is only a single add to cart form. This part could be tricky if you are not comfortable with editing the html directly. You need a single form with a single submit button.
After you modified the forms so there is only one you would need to modify the add to cart server behavior code. It would need to have the triggers for each updated to be based on the posted quantity for that size. This way the sizes for the items will only be added based if they have entered a quantity.
After all of this you would need to check the redirects that are on the page and make sure that the eCart redirect code only occurs after all of the add to cart code.
The trickiest part of this is combining the forms and editing the code for the add to cart server behaviors. If you are interested in going this route you should first get a single add to cart on the page working, then once it is working you can add another and combine them. If you can combine two on the page combining more should not be a problem. This is a custom feature and will not be supported itself but if you need any advice or pointers on how to get things going please post back and we will be glad to help you with whatever we can.