On a related note, you can easily find the domain of the B-spline nonrigid transformation by doing

grep domain YOUR_WARP_XFORM_FILE

You should be seeing a vector of 3 floats, and the domain of your transformation is the region between (0,0,0) and that vector. Coincidentally that should match almost exactly the coordinate domain of the fixed image you used to create the transformation (although with some settings of warp(x) the transformation domain could potentially be quite a bit larger, but it should never be smaller).

Torsten