Megalophobia

Megalophobia, Also known as macrophobia is the fear of large things. This could be due to the fear that a large object might collapse or fall. Another reason is the intimidation of the size of the object. Megalophobia is also common when it comes to oceans, and underwater, typically with large ships / sunken ships or large structures above or beneath an ocean in general.

Related
Microphobia - Fear of small things