What’s the difference between "in" and "at" when used before a Location/Site/Country/County etc

We always were told that you could use the word in before a place which is a large space e.g. country/city etc. Whereas, before a smaller site or place you should use at. But actually I don’t know what the precise scope limitation between those two words is. Large? How large is large? Small? How small?

Solution 1:

At is used to talk about the position at a point.

Examples:

It's very hot at the center of the Earth.

Turn right at the next traffic light.

Sometimes we use at with a large place when we consider it as a point that exists on a journey, as a meeting-place, or as the place where something happens:

You have to change trains at Didcot.

Let's meet at the station.

In is used for position in a three-dimensional space (when something is surrounded on all sides):

I don’t think he is in his office.

Let's go for a walk in the woods.