You may need to force specific hardware to be simulated for your application (e.g. continuous integration). There's easy way to do it in the command line:
$> defaults write com.apple.iphonesimulator \
"SimulateDevice" '"iPhone (Retina)"'
This will make iPhone Retina the default hardware.
Note: Simulator application has to be restarted for the change to take place. Relaunching iOS application via Xcode is not enough.