What is a good size tank for narwhals?

No one knows, because no narwhal has ever survived in captivity. Obviously, if you are looking to try to keep one as a pet anyway, it would have to be much bigger than a narwhal. A narwhal is a whale, so perhaps you should try for a tank similar to those that zoo keepers stick orca whales in. However, one needs to consider how far a narwhal travels in the duration of one day. Even orca tanks aren't really big enough for an orca's happiness.