Sunsynk 8kw inverter. 

What size wires should I use for the Earth and Neutral bridge wires going to the relay? 

the switching wires from the inverter or the actual earth and neutral wires?

for the earth and neutral I would say no more than 2.5mm^2, it should only ever need to carry the trip current for your earth leakage... 30mA? so smaller should also be ok, although I've seen some recommend that it should be able to carry 60A, which I think is nuts... after all, the earth leakage should put its foot down loooooong before you even reach 1Ampere...

for the switching, whatever the relay will consume when switched, probably 1mm^2 would do, I'm guessing, check on the specs for the relay/contactor...

 

Thanks. I'll use 2.5mm2