Any size you want. You either have one or not. Some places will accept portable units, some won't.

I made one using a 20 litre Collapsible water container that can connect to caravan with a 900mm length of pool filter hose (Kreepy Krawly), has correct size spiggot hole to fit hose and carry handles - simples.

What you need depends on how much grey water you create and how long it will be before you leave the area and are able to empty it.
CMCA require a MINIMUM of 40L if my memory serves me right.
Long hot showers every day for a week and you will need a BIG grey tank.
Cheers,
Peter

Most portable caravan black waste tanks are approx 20 -25 litre - Waste water tanks range between 20 -40 litre. A big consideration, particularly for elderly van owners is the weight of full tanks. Even tanks fitted with wheels need to be lifted into car, to empty, etc.

Averaging 1Kg/Litre - There is also the consideration of; where do you carry the tank whilst travelling?

Remember a 10 or 20 litre tank can be decanted into a drum until disposal.

My expandable tank can fit under a seat in tug (when empty), weighs less than 1/2 Kg - With regard to Dabbler's comment; I have NEVER been refused entry to any park, camp or property.

I have found many members of CMCA extremely elitist and contemptuous towards caravans - CMCA will accept membership fees from caravan owners, but disallow voting rights. I therefore don't regard their input into my lifestyle choices.

We opted for a 90 litre grey water tank because we have 2 x 90 litre clean water tanks. We figured that half capacity of total water should be ok and in practice it has been ok. We keep it empty most of the time and only travel with it partly full when there is no acceptable place to empty it. Only had it near full once when we couldn't empty it for a few days.