Are you saying the calibration cube is printing at 50% of the size it is supposed to? If so, first make sure you didn't scale it down accidently when you sliced it. But if not, you can try changing the line:

#define DEFAULT_AXIS_STEPS_PER_UNIT

in your configuration.h file. There will be 4 numbers on the line. X,Y,Z & E. If you double each of the X,Y & Z numbers, the print should scale up to be twice as big.