$_product->getTypeInstance(true)->getUsedProducts ( null, $_product);

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/>';
 }

$_product->getTypeInstance(true)->getUsedProducts ( null, $_product);

foreach ($_product->getTypeInstance(true)->getUsedProducts ( null, $_product) as $simple) {
     $objectManager = MagentoFrameworkAppObjectManager::getInstance();
     $StockState = $objectManager->get('MagentoCatalogInventoryApiStockStat‌​eInterface');
     echo $simple->getName()." with size ".$simple->getSize()." have a stock of " . $StockState->getStockQty($simple->getId(), $simple->getStore()->getWebsiteId());
     echo '<br/>';
 }