Which variety of Willow is it? There's a big difference in size between Salix babylonica, Salix caprea (Kilmarnock Willow as a standard) and Salix hakuro nishiki. Obviously, if you've got a tree that gets 100 feet high, the roots on that will be much bigger than on a dwarf form. Even so, you can assume the roots will extend more than a foot or three beyond the drip line - as for how deep, hard to say, willow roots tend to head for wetter parts.

I would say put it in a container of some sort. Then you can take it out every two years and check the roots. They could be trimmed then if you want to.