Oort cloud is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to the oort cloud.
Earth is 150 million kilometers from Sun. This is called 1 AU.
Imagine a sphere, as wide as Earth's orbit. It's big. So big, that light will take ~ 17 minutes to get from one edge to the other. Let's call it an Earth sphere.
Neptune orbits 4.5 billion kilometers away from the Sun. A sphere as wide as neptune's orbit (or Neptune sphere, if you will), would contain 20,000 Earth spheres.
The Oort cloud, a spherical region full of comets and icy bodies, starts from ~2000 AU away from Sun, and extends as far as ~100,000 AU. (This is an estimate).
At that size, it would contain 27 billion neptune spheres.
It's so large, that light will take ~3 years to get from one edge to the other.