Hello,
If the Normal style, that is the default style for paragraphs doesn't have a size value, then default document styles are used.
You can get the default styles from a DOCX using Indexer:
$indexer = new Indexer('document.docx');
$indexerOutput = $indexer->getOutput();
// default run-of-text styles
var_dump($indexerOutput['styles']['docDefaults']['rPrDefault']);
// default paragraph styles
var_dump($indexerOutput['styles']['docDefaults']['pPrDefault']);
You just need to get the w:sz value (the size value is usually found as default run-of-text style).
Regards.