Ignoring the practicality of doing something like this, would it be possible to make a large scale shaped charge using nuclear weapons, or is there some limitation that makes this only possible with small scale explosions?
Yes, a nuclear "shaped charge" is possible. You can think of a conventional shaped charge as a set of small explosive charges in a particular 3D arrangement(e.g., a conical shell). It is the arrangement of the charges and the relative timing with which they are triggered that determines the shape of the emerging shock front. An array of small nuclear bombs, triggered with specific relative timing, can produce a non-spherical shock front just as a shaped charge does.