How great is God?
No one really knows this. Or else. Our Lord's greatness is not understood by any human mind, no computer with the greatest computational power. To put it more figuratively: the Divine being goes beyond any framework we could impose.
God is the creature who created everything. So it is greater than anything he has created. Greater than anything that has existed or will arise in the future. To quote the lyrics of the song: How great is God? He has time in his hand. He lasts forever and ever. He is the beginning and the end.
At the same time, each of us will perceive God and his greatness a little differently, because he is everything we can imagine, but at the same time he is something much, much bigger. So this subject (as with most subjects concerning God) should be approached quite individually.
I refer those interested to the song that was mentioned earlier. The song is called "How great is God?" and is a glorifying song for our Lord.