Android does not provide a good solution for this. Either creating combination qualifiers and duplicating images, or rolling something yourself (programmatically getting the appropriate resource using Resources.getDrawableForDensity
or similar) is your best bet.
It would be really great if there was a system where you could provide many images at arbitrary pixel sizes, and Android would just pick the closest one based on the desired size. But it seems the resource loading system as it is is only designed for the drawables themselves to dictate the size.