I believe it depends on the output device. For example, to make a PNG file higher resolution you would just make the chart bigger. I've had real good success with EMF files in newer versions, avoiding the need to use raster graphics in word documents at all.
If you can't set it directly, I presume there is a way to figure out a transformation between the size of the plot and the DPI (my V15 does not appear to have any options to control DPI directly).