What is the biggest facultative anaerobic metazoan known to science?
I think a tapeworm is your best bet. For tapeworms specifically, I found this textbook, Animal Physiology: From Genes to Organisms, which claims:


  Active facultative anaerobes include some parasitic worms such as tapeworms that can survive without oxygen indefinitely...


I was not able to find any primary research on the subject either @WYSIWYG (other than a paper from 1951 that doesn't seem to be available anywhere), but if the textbook's claim holds, I can't think of anything larger than a tapeworm.