What do you mean by the phrase " they chorused as big as a battleship ??

Dear Student, 

The narrator and his family set out to sail for Australia. However, in the midst of their voyage, they had to battle severe storms. Sea-waves wrecked their ship. So, their ship was in no condition to reach Australia. For safety, they were trying to reach the nearest island called Ile Amsterdam. So, when the daughter and son of the narrator realized that they had reached there, they shouted together that they had found the island, which was as big as a battleship. 

Regards,