in a form element, the value is enclosed by quotes, so the quote in the record is closing the value prematurly
change:
value="<?php echo((isset($_GET["invalid"])?ValidatedField("productsupdate","ProductSize"):"".$row_WADAproducts["ProductSize"]."")); ?>"
to:
value="<?php echo(str_replace("\", """, (isset($_GET["invalid"])?ValidatedField("productsupdate","ProductSize"):"".$row_WADAproducts["ProductSize"].""))); ?>"