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.
We inherited a 23l Fiamma with our secondhand camper. Haven’t used it yet. Our alternative would have been a plastic jerry or a plastic drum.
As for capacity, prior to the camper’s ensuite we used a pop up ensuite tent with a 12volt shower in a 9 litre bucket of warm water. Not a luxurious experience but 9 litres got the job done.