Huge; big — a colloquial way to say something is large; can also be used for emphasis or surprise.
この魚、でかいね！写真に収まらないよ。
This fish is huge! It won't even fit in the photo.
でかい声で叫ぶと、周りの人に迷惑だよ。
Shouting loudly is a nuisance to people around you.
夢がでかい人は行動力もあることが多い。
People with big dreams often have the drive to act on them.