It is often difficult to grasp. This is why when you look at an empty piece of land, it can sometimes be difficult to visualize actually fitting a building on it. And why when you look at an empty room, it's common to think, "there's no way that furniture will fit in here." But in the end, it does fit.

It also tends to be relative. Here in North America, it is common to argue over things like parking space dimensions and drive aisle widths. We'll say things like, "well, people like their big cars." But then you travel to Europe and you find streets like this:

And this:

The first is only marginally bigger than the width of a parking space in Toronto (2.6 meters). And the latter is only marginally bigger than the width of a typical two-way drive aisle (6 meters). So are these too small? Well, it depends on your perspective.

If your basis of measurement is the size of cars, then these streets will seem too small. Cars also keep getting bigger, so you have this inflation factor to deal with. But if your basis of measurement is something else, such as walkability, then maybe they're just right.