The Essence
Al-Kabeer is the Incomparably Great. All greatness belongs to Him alone, and everything else in existence is insignificantly small in comparison to His absolute power, essence, and authority.
In the Quran
وَأَنَّ اللَّهَ هُوَ الْعَلِيُّ الْكَبِيرُ
"And that Allah is the Most High, the Grand (Al-Kabeer). (Surah Al-Hajj 22:62)"
In Your Duas
اللَّهُ أَكْبَرُ كَبِيرًا
"Allah is the Greatest, very Great."
— Sahih Muslim 601