In FreeCAD, one would first need to first Create shape from mesh..., and then scale it in Draft Workbench. For details of how to do this, see Lesson 1 and Tutorial 24 at https://wiki.opensourceecology.org/wiki ... Tutorial_1.

For the scale factor, I found that the scale factor from SweetHome3D to FreeCAD is exactly 10x by taking the box in SweetHome3D (3'x3'x3'), which appeared as 3.6" in FreeCAD. This result is consistent for other objects.