Two ducks can fit in at a time (though it is tight) and they are able to splash water over their whole bodies. They snortle and chortle (and sometimes mate) in the tub and then jump out and spend a good amount of time preening and fluffing and re-oiling their feathers with oil from the gland above their tails.
I give them fresh water daily and the tub is small enough, I can just kick the dirty water over and it waters the raspberry patch and doesn't cause much of a mud pit. It really keeps them happy and clean. For me, the pros of this system are:
1) Their water stays clean since it is so easy for me to change it
2) The water isn't getting wasted...it is watering the raspberries (and fertilizing them)
3) It's good enough for now. Small water is better than no water or dirty water.
Actually..... you can go a little smaller than a wash basin....in the winter a large dog bowl is even easier to fill when hauling water from the house is less than fun. They seem pleased enough with it.