What is smallest city in this country?

It would be a tad more helpful if we knew which country you were talking about. However as the question is in English, the smallest city in England is interestingly the City of London, with a mere population of 7,185 (according to the 2001 census). If you don't believe me, check Wikipedia's list of the UK's smallest cities. The smallest in England is the City of London. Fact!