Welcome to GoY.
wow that is a bit big to go in a pot really. Which Salix is it as some make trees 30ft tall. Salix are thirsty trees. so the answer is the biggest you can find. But to be honest it would be much happier in the ground.

Which variety of Salix is important to know to give a good answer... if its a standard Kilmarnock Willow, or a standard Salix integra hakuro-nishiki, that's different from, say, Salix tortuosa in terms of pots versus ground and space it needs