How we can select multiple sizes of configurable product from product details page? Also, Can we add configurable product different size prices simultaneously in Cart?
You can't, not with configurable products. But if size is your only option for the product, grouped products might be a better option. They provide a quantity input per option.
You can read more on how to create grouped products here: http://www.magentocommerce.com/knowledge-base/entry/tutorial-creating-a-grouped-product
I am not sure how you actually code this, but I have used an extension. As a for example only, this one, although there are many others if you search for
magento configurable products grid
With the drop down list, it's very difficult to add configurable product different size prices simultaneously to cart as your requirement. Therefore, you need to find another way to display configurable products which can show all simple products of this configurable product and then customers can choose quantity for each one.
Some kinds of display that you can consider are a grid table or a matrix form. All supports showing children products in a convenient way to add multiple products to cart at one time.