For 2 orandas I'd say you're looking at around 120l minimum and an external filter rated for at least twice the tank volume, as they produce a lot of waste. Others on here know more about goldfish than me, though, so you may get a more accurate answer from one of them

 

I'd go further and say get even bigger than 120L - if you can afford it, more like 200L. You don't want to find your fish are stunting further down the line and have to choose between shelling out for another tank or re-homing them (they are not suited to living in a pond). They can indeed grow very big - 20-30cm / 8-12 inches (they have been recorded as bigger) and being round bodied fish with high waste output you will want to do what you can to avoid problems (hence also the double volume filter).

 

Just to reiterate what Phoenix has said, orandas are one of the bigger types of fancy goldfish, so a tank closer to 200L would be best for a pair

 

I think 200l tank may be sufficient for a pair of Oranda.