Here’s the Mars Goliath coil: 15 inches, DD. It’s one of the largest coils which exist for the Garrett AT PRO metal detector. It allows hunting with full sensitivity without false signals. Watch the video: practical search with Mars Goliath coil.
Posts on Garrett AT PRO (photos, videos, comparisons, tests) are here. Also, on Knowledgebase pages there is a complete list of coils for Garrett AT PRO, an instruction manual, as well as everything you need to know about the metal detector before buying it.