It used to be twice the size of a picture. Now?
This problem is estimated to wait until you get the iPhone 6 plus real machine to tell you that, after all, the simulator is different from the DPI of the real machine, so you can only make do with it.
Please refer to Apple’s official documentsiOS Human Interface Guidelines: Icon and Image Sizes
And another article that vividly explains the screen differences between generations of iPhonesiPhone 6 Screens Demystified