The options you’re looking for are ViewPortIn and ViewPortOut, which are part of the MetaMode attribute string.
Scaling is enabled if the sizes of ViewPortIn and ViewPortOut don’t match. nvidia-settings has options to specify the scaling explicitly in the advanced display config tab.
External Media
In basic mode, there are also entries marked as “(scaled)” in the resolution drop-down. Those automatically configure ViewPortIn scaling for common screen sizes that aren’t natively supported by your monitor.