Wikipedia has more on 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 (Submechanophobia) or large structures above or beneath an ocean in general (megalohydrothalassophobia).
The term "megalophobia" comes from Greek roots: "megas," meaning "large" or "great," and "phobos," meaning "fear." People with this phobia tend to be scared of very common large objects such as large machines, boats, animals and more
Since megalophobia can be tied in with many other phobias (thalassophobia, mechanophobia etc,) it's hard to diagnose properly. However, one way to know that you. have megalophobia is to see if the source of the fear itself is the huge object. Some phobias might be of something that happens to be large, but the size is not the cause of the fear, as an example, if you're afraid of the ocean, which is very large you probably don't have megalophobia but thalassophobia instead
Related[]
Microphobia - Fear of small things