Using the dropdown in the 'Insertion Scale' section select 'Meters' if not currently set.Type 'units' in to the command bar and press enter, the Drawing Units window appears:.Open the DWG file downloaded from Digimap in AutoCAD.The instructions below explain how to re-scale a drawing from metres to millimetres.
AutoCAD typically deals in millimetres, but Ordnance Survey data available in Digimap is provided in metres.