FHD(Full HD) is 1920*1080.
Which the res folder would be used in FHD and what's the dpi value should be set?
The Smallest width
is configured by manufacturer, so it may be varied. For instance, Nexus 7 and Kindle Fire HD both have 1280x800 screen resolution, but the smallest width of N7 is 600dp, while Kindle Fire HD is 533dp.
Here is a way to calculate this:
If 480x800 is mdpi(160) for 5 inch device, the screen dpi(screenDPI) of the device with resolution 1080X1920 is 160*(1080/480)=360 (NOTE: manufacturer might change this number). Then the smallestWidth = 1080/(screenDPI/160).
Use Configuration to find out all the information of the device.