Huge
Gigantic, Enormous
noun, な adjective
A giant big thing is something that is huge and gigantic.
きょだい
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.
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.