$_product
is your configurable product.
To get all its simple products use :
$_product->getTypeInstance(true)->getUsedProducts ( null, $_product);
So you might have something like :
foreach ($_product->getTypeInstance(true)->getUsedProducts ( null, $_product) as $simple) {
$stock = Mage::getModel('cataloginventory/stock_item')->loadByProduct($simple)->getQty();
echo $simple->getName()." with size ".$simple->getSize()." have a stock of $stock";
echo '<br/>';
}
I let you adapt to your precise needs and ask question if needed