Schottky diode for MOSFET H-bridge
how can I calculate the value of the schottky diode?

There are no schottky diodes in your circuit: -

If however you meant the 4 diodes I've ringed in blue circles then there is no need for them because each N channel MOSFET has a bulk diode inside the chip that performs exactly the same function as those external 1N4001 devices (D1, D2, D3 and Another D3): -