If you're aware that this will reduce the rendering quality of the iOS Simulator window, you can open up your Terminal and scale the simulator, e.g. to 40%
defaults write ~/Library/Preferences/com.apple.iphonesimulator SimulatorWindowLastScale "0.4"
To change the current scale, just use the menu commands in "Window > Scale" or press cmd+1, cmd+2, cmd+3
.
Edit:
Quit the Simulator before applying this setting
(it's the last scale used, so quitting and updating the preferences is the main trick here).
Tested with Xcode 6 on Mac OS 10.9.5 / iOS 8.0.2 SDK
and Xcode 6 on Mac OS 10.10 / iOS 8.1 SDK
Edit2:
While it did not work on Yosemite a couple of times, it just worked fine again. Maybe a reboot is required now and then.