They told ya it was only supposed to grow to 40"??? Gotta love some nurseries! I have seen many which grow larger then that. The shrub is almost at its max, but may still grow another foot or 2. They do prune back nicely though, in case you wanna go that route.

Yes willows are easy to do clippings from. It is best to take an end from a nice actively growing branch and cut it on an angle, take that clipping and dip it into a rooting hormone and then put it in water, and keep it in a sunny location. Change the water every couple of days, or it will start to build up bacteria which will eat at the new roots as they appear.
It will send out new roots in a few weeks to a month. Once it has developed a fair amount of roots, you can plant it in the ground at that point or move to a pot, (to keep for the winter) and transplant in the spring if you are from a colder climate.

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Mike Maier
aka
The Plant Doc