8 litres
The 4 is still not available here.
As for cleaning: when they are empty, more or less, I rinse them with warm water to get rid of whatever trub.
Fill again with water. add lid, shake and drain
They work well, but are a bit flimsy.
If you fill them totally and drain by holding them upside down, they look like they collapse. They don't, but better to empty them slowly.
Also: first 2 I got came from a shop at 2400 mtr altitude and I am at 400, so they looked imploded. Didn't do them any harm, but when I bought the next 2, I asked them to put a little pressure in them before shipping.
It's a very easy size to manage and I make small batches anyway.
If I remember correct,
@Mark Farrall also has oxebar kegs
I wouldn't mind a couple of 4 litre ones.