I think, from looking at there downloadable firmware the Z size refers to Z max length and should read 200, not 20.
So, look for these lines
#define ZAXIS_STEPS_PER_MM 400 (might be set to a different value atm)
#define ENDSTOP_PULLUP_Z_MIN true
#define ENDSTOP_Z_MIN_INVERTING true
#define MIN_HARDWARE_ENDSTOP_Z true
#define Z_MAX_LENGTH 200 (might be set to a different value atm)