So objects appear 50% smaller? Sounds like the microstepping jumpers under your stepper drivers are set incorrectly. Pretty sure they should be set for 16x. Please check, or increase the x y and possibly z steps per mm to roughly 2x what they are.
On the other hand, If you only have access to 50% of the bed, but objects are printing the correct size, you’ll need to adjust the bed size definition In the configuration.h file to 500×500.