Hello,
Using the Indexer class (http://www.phpdocx.com/api-documentation/docxutilities/indexer-parse-word-documents-with-PHP), you can extract the existing images in a DOCX and using GD functions from PHP (http://php.net/manual/en/book.image.php) get their sizes and dpi.
To get the same size, please add images with the same size (width and height) and dpi.
Regards.