Hi Simon,
When importing a dwg/dxf file, do not miss the import options (bottom right of the file browsing window when importing). There you need to set the correct unit the original file was modelled in.
https://lh3.googleusercontent.com/-utZuae1KFxs/T0DQkqaoElI/AAAAAAAAHz8/t3pHK9BgUjs/s747/dwgdxf.png
Also, when you are at it, some dwg/dxf files contain geographical data. Best is to uncheck "Preserve drawing origin" below the units settings as SU may import the cad file many kilometres/miles from the origin thus causing serious head-aches with clipping and other issues.