We will never be able to understand and comprehend it, because the whole universe compared with Heaven is like a drop of water compared with the ocean...

However, the Bible makes a beautiful description of it for our imagination:

1- It is a gigantic, enormous city:

As big as the whole world known at that time, the whole Europe!... and tri-dimensional!:

"I saw the Holy City, the new Jerusalem, coming down out of heaven from God... The angel who talked with me had a measuring rod of gold to measure the city, its gates and its walls. The city was laid out like a square, as long as it was wide. He measured the city with the rod and found it to be 12,000 stadia in length, and as wide and high as it is long." (Rev.21:2, 15-17)

12,000 stadia are about 1,400 miles, 2,2000 kilometers... The length of the whole world known at that time!... and the same length in high!... and it is not plane, but tri-dimensional... going up another 1,400 miles!...