The big sky isn't just any old patch of sky. It's the skies above — the open sky where birds soar, the heavens where stars twinkle.

You already learned 空 for "sky," right? 大空 basically means the same thing, but with a more poetic sense of grandeur and scale. You won't hear it much in everyday conversation, but you'll encounter it in things like literature, song lyrics, and slogans or taglines for aviation-related media.

The readings for this word are both kun'yomi readings. You've learned both of these readings by now so you should be able to read this on your own. Just remember that そら becomes ぞら because of rendaku here.