hi,
in Sketchup import --> click the 'Options' button;
this allows you to set the units for import.
If an ACAD drawing is drawn in 'inches', Sketchup will import to correct scale even if you've set your import units to 'Meter'.
However, if your ACAD drawing is drawn in 'unitless', then Sketchup will take 1 unit in ACAD drawing as whatever unit you've selected.
Hope this helps. 😉