How do I answer 这是哪儿?
It really should depend on the context. Note that "Where am I?" has similar ambiguity in English.

For instance, if I'm at a shop and speaking to someone on the phone who wants to know where I am, and I ask the shopkeeper 这是哪里？, it's expected for the name of the shop (or its location) to be given. This might also hold if I am on the street.

Comparatively, in some cases, I might want less specific information. If someone is bringing me into an unfamiliar store, I might instead expect the answer to indicate what type of store I am in. If I have been driving long distances and have just stopped, I might expect 这是哪里？ (if asked by a passenger I am carrying) to be about what town I am in, not what road I am on.