In a BBC Radio 4 interview a couple of days ago, the commanding officer of HMS Echo said the search area had been localised to 10 miles by 5 miles. Assuming nautical miles, that is about 172km2.
In other words about the same size as the island of Jersey.
Quite a big area to lose a ULB on its own but small enough not to lose an entire aircraft, or most of its wreckage.