How big can a dog fish get?

You're likely referring to the dogfish shark, not a "dog fish". There are actually several species of dogfish sharks, and their size can vary considerably:

* Spiny Dogfish: These are the most common type of dogfish. They typically reach around 4 feet (1.2 meters) in length. The largest recorded specimen was 5 feet 8 inches (1.73 meters).

* Greenland Shark: These sharks are much larger than spiny dogfish. They can grow to an average length of 12-14 feet (3.6-4.3 meters). The largest Greenland shark ever recorded was a whopping 18 feet (5.5 meters) long.

* Other Dogfish Species: There are several other species of dogfish, such as the smooth-hound and the lesser spotted dogfish, that are generally smaller than the spiny dogfish, typically reaching lengths of 2-3 feet (0.6-0.9 meters).

So, the answer to your question depends on which species of dogfish you're interested in. The Greenland Shark is the largest, reaching lengths of over 18 feet, while the spiny dogfish is the most common and typically grows to around 4 feet.