Step 6: Final dimension check
In the main window of Carrara I could check that the vertex model scaled at exactly 4 cm wide. As it turns out, however, Collada (the mesh interchange format I meant to use) does not have a global standard. My version of Carrara exported a DAZ version of Collada that Shapeways would not accept.
It turned out the best path was to export in Wavefront "obj" format, and then convert that in Hexagon (another DAZ modeling application) to .stl -- however, some peculiarity made the objects come in at about 1/2 the proper size.
Fortunately, again, I knew the model had to be exactly 4 cm wide. So I re-scaled in Hexagon, and exported in .stl with "millimeter" selected. Then used the Shapeways importer set also to mm.
The last step was to check in my basket at Shapeways, where the quote is, to see if they listed the object at 4 cm x 1 cm. After a few false starts, that is how it listed.