- He lives at Ottappalam in Kerala. (Ottappalam is a village in Kerala.)
- His brother lives at Mumbai. (Mumbai is a big city.)
- Turn right at the next corner.
- She grew up in Africa.
- Let’s go for a walk in the woods.
- He is in the office.
This rule is not very rigidly followed, and in is often used for small places too, though at is seldom used for big places.
Sometimes we use at with a larger place, if we just think of this as a point on a journey or a meeting place, for example.
- The planes stops for an hour at New Delhi.
- Let’s meet at the club.
At is particularly common with proper names used for buildings or organizations.
- She works at Legal and General Insurance.
- She works in a big insurance company.
- I first met her at Harrods.
- I first met her in a shop.