Create four new folders in the Objects/res folder:
(a) drawable-xlarge (7" to 10"+ devices)
(b) drawable-large (4" to 7" devices)
(c) drawable-normal (3" to 4.25" devices)
(d) drawable-small (under 3.25")
Create an icon of each size and put in the appropriate folder.
Once in the folders, give each file the same name as the one (if any) in the "drawable" folder