Create MKCoordinateRegion
object and set map view region for that:
CLLocationCoordinate2D location = CLLocationCoordinate2DMake(<LATITUDE>, <LONGITUDE>);
MKCoordinateRegion region;
// <LATITUDE> and <LONGITUDE> for Cupertino, CA.
region = MKCoordinateRegionMake(location, MKCoordinateSpanMake(0.5, 0.5));
// 0.5 is spanning value for region, make change if you feel to adjust bit more
MKCoordinateRegion adjustedRegion = [self.mapView regionThatFits:region];
[self.mapView setRegion:adjustedRegion animated:YES];
Edit:
As you mention in your comment that you need dynamic city enclosure. In that case we need map zoom level (an integer value - default is 0) .
That means some API or web service which return's city/co-ordinates and zoom level. So that from that zoom level we can achieve map span/region calculation.
And a link to get zoom level from lat/long: http://troybrant.net/blog/2010/01/set-the-zoom-level-of-an-mkmapview/