I've seen earlier posts that probably are related to what is happening to me.
I've exported stl files of an object from FreeCad and from Blender, both programs reporting units in mm. Upon import, Simplify3D says "Can not auto arrange. Largest part exceeds build dimension." However, the Object Size Dimensions (Model Settings) reports the correct dimensions, which will fit on the print bed. I don't see how to work around this problem as accuracy is important.
Any ideas?