As small, meager human beings, with the limited mental and emotional capacity we have, we cannot completely comprehend God and all of his capacity. However, we can see his work, we can talk/pray to Him, we can have a relationship with Him, and we can recognize his holiness and greatness.
I try to think of God as pure greatness and holiness. This is nearly impossible to put into words. It's like, have you even been to the Grand Canyon? I have, several times. Every time I've been there, I stand on the edge and look out into the vastness of it. I feel so small and insignificant. Now, I attempt to multiply that feeling by 100, by 1,000,000, by 1x10^1,000,000, by 1-with-a-hundred-million-billion-zeroes behind it...etc, etc. God is infinitely greater than I am.
God is love. Think about that hard. He IS love. And he loves us so much he sacrificed his OWN perfect Son, to be killed on OUR behalf. How awesome is that love!?
We are beings in the "real" world/dimension/realm, but we also have souls. Our souls are in the spiritual world/dimension/realm. The spiritual realm is unseen by our eyes, but we can feel it in our heart--that is to say, in our soul.
God is in the spiritual realm and in the "real" realm that we know simultaneously. Another way to think is to contemplate your fear. For instance, what do you FEAR more than anything? Or, have you ever been scared? I mean REALLY scared. Like where you feel your heart weighs a thousand pounds and is about to thump right out of your chest and you have to gasp for air? Think about this verse, Matthew 10:28, in which Jesus is talking about being fearful of God:
"And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell."
I mean REALLY think about this: God can save or allow your soul to be destroyed if you refuse to acknowledge Him. He knows your deepest secrets. He knows every lie you've ever said; even to yourself. You cannot hide from God. He KNOWS you. That's some scary stuff if you really stop to think about how powerful he is!
I mean, he made me, he made the world, he made the DNA segment that organizes the 316,800,000 feet of blood vessels than run inside my body! How AWESOME is THAT!
God thought-up the Tyrannosaurus Rex and the electron and the Penguin. Did you see March of the Penguins? Pretty amazing what those creatures do to stay alive. God thought them up! How awesome is this God we barely know!
In the words of Bill Murray from the hilarious film What About Bob:
"He is so far above us, we're like ropes on the Goodyear blimp."