Huge
Gigantic, Enormous
noun, な adjective
A giant big thing is something that is huge and gigantic.
巨大 is often used in writing to describe something enormous.. It can be for anything of immense scale, not just physical objects like buildings or monsters, but also abstract things like the impact of a natural disaster or even a person's influence.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
なんて巨大なピーマンなんだ！
What a huge green bell pepper!
巨大なトーフグ人形がオフィスに届きました。
A gigantic Tofugu doll has arrived in the office.
日本に旅行してる時に、南海トラフみたいな巨大地震が来る可能性もあるでしょ？
There is a possibility of a huge earthquake, like one from the Nankai Trough occurring while you are traveling in Japan, right?
彼はとっても忙しい巨大企業の社長さんなんだから、アポの約束は絶対に早目に取っておいた方がいいよ。
I strongly suggest that you schedule the appointment with him well in advance because he is the president of an enormous company and he is very, very busy.