Geeetech

Hello I have a Prusa Reprap Acrylic i3 it had a sanguinololu board but now I want to replace it with a GT2560 board ...what firmware should I flash the Gt2560 to work with the printer ??? please help....Thanks.

Use the one for the I3X single extruder printer, and note the dimensions here:
// Travel limits after homing
#define X_MAX_POS 210
#define X_MIN_POS 0
#define Y_MAX_POS 210
#define Y_MIN_POS 0
#define Z_MAX_POS 180
#define Z_MIN_POS 0

Also the steps/mm here:
#define DEFAULT_AXIS_STEPS_PER_UNIT {78.74,78.74,2560,95} // MXL, Z M8=1.25, MK8

I think that might be enough.

I did all that ,now my prints come out real small?? I did not change anything in Repetier as it was working great with the other board.

also it wont print in the center, it prints on the side of the bed???

now my prints come out real small??---have you changed the steps to those that in your original firmware?
And the bed center, you can try to calibrate it here: