If you want to comply with the Leave No Trace protocol established by the CMCA and negotiated with various Councils and LGAs for free camping, I think you need minimum 35 litres for two people in a van with a shower. So go the 40 Litre. You don't have to carry it full, just empty it ASAP to keep the weight down.
ALso, AFAIK it doesn't have to be fixed. You can capture GREY water (not black) in jerries if you wish, just as long as it doesn't overflow. Buckets are not an acceptable alternative to jerries.