600mm deep. if its single phase 4mm2 should be plenty big enough for a few fluoros and a powerpoint or 2. take it to your switchpanel, not the closest powerpoint in your house and all will be fine. unless you have a massive backyard. then go 6mm2
Put a 16 amp breaker in the board and a 16 amp combo RCD in the shed.
RCD's save lives.
If you're just running the conduit, then 32mm ORANGE will be fine.

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Balls.