But we do have a pretty good idea where it starts and stops, based on years of surveying using the most advanced technology for geophysical survey, plus site characterization, and also many many archaeological excavations. We are always finding new information, but all in all the site is about a half a square kilometer.
Here's a link to the interactive map of Heracleion
Many of the artifacts that Franck Goddio's team have found will be traveling the world with the exhibition Osiris the Mysteries of Sunken Egypt