The vastitude of space is its immensity: We don’t even know where its limits are. The vastitude of the ocean hits you when you're sailing in the middle of the Pacific, with only water as far as the eye can see.
Vastitude is usually used along with words that represent spaces, areas, or regions rather than objects. For example, you won't often read about the vastitude of a mountain, but you'll read about the vastitude of the plain you can see from the mountain. In addition to size, the word can also refer to the area or region itself, as in "The tundra was a vastitude he had glimpsed only from an airplane window." It's sometimes used figuratively, as in "The vastitude of some people’s ignorance is astonishing."