And before you jump in with 5 litres... are you sure?
The reason I ask is that I filled up two petrol cans last week and somehow managed to get 6L in each one. I'm pretty sure this is probably down to the calibration of the petrol pump but I thought I'd double check before complaining to the petrol station.
If they do hold more than 5L then I've been doing my fuel mix wrong for the last seven years! Hmmm....