Sub panel wiring

Hi there
I have a electrician supplying power to my shed he has used 6mm twin and earth to a sub board. The sub board has a 63amp breaker with 3 rcd mcb combos two 20amp for power and one 10 amp for lights I'm a bit concerned on the size of the neutral wires supplying the rcd mcb's from the nuetral bar in the sub board, what size should these wires be

Thanks in advance