I've done some significant modeling in AutoCAD and inported them into Blender using 3ds as an intermediary. How accuratley does blender convert files when it imports them? It seems things are growing and shrinking.
When importing objects into Blender, you need to be aware of a) which axis is up (y or z, usually -- Blender uses z) and b) what world units you're using. Blender defaults to "Blender Units", but can easily be switched to either Imperial (inches & feet, etc.) or Metric (centimeters & meters, etc.). To do so, go to the scene panel >> units section and select whichever units you prefer to use there.
If that doesn't work, you may simply need to rescale the image and then use ctrl/cmd+A to reset the scale.
And if that doesn't work... then there's a trick to it I haven't come across, yet.