To say "size" is to speak in the physical, and He is not physical, He is Spirit. No one understands what that really means. However, metaphorically I think of the Spirit as the point. The point of Scripture, the "why" it is written, is the spirit of the Scripture. A point in geometry has no measurement. But two points together make up a line with an infinite number of points in between. Likewise, a moment of time corresponds to a point in geometry. A moment of time has no measurement, but two moments create past, present and future.
He is a Spirit and by His Spirit He created the cosmos. Logic states that Creator is always greater than the created. Is greater synonymous with bigger size? No.
Regarding the spirit world of Heaven, scientists say that the weight of the universe is 90% more than the universe that can be seen. So they make up a substance called "dark matter" to account for the weight. I have no idea how they can "weigh" the universe. However, there is no need to make up something to account for the weight. I believe that unaccounted for 90% is the spirit world. We believe angels, which are spirits, were made before the Earth was made, therefore the spirit world was made before the physical world. And I am convinced that the spirit world and the physical world are different dimensions of the one creation.