The capacity will be between 10 and 20 litres. But, as the dealer says, it's impossible to predict any one driver's usage rate. What happens is that when there is enough AdBlue for 1500 miles average use, a light comes on warning you to refill the AdBlue tank, and that gives you plenty of time so no need to carry a container in the car. Best to use 3.5-litre or 4.7-litre containers with a spout. 10-litre containers without a spout make a very messy job of it and you have to get any spilt AdBlue off the paint promptly. Also, wash your hands. More on Adblue: heycar.co.uk/guides/what-is-adblue