diff --git a/IPython/extensions/rmagic.py b/IPython/extensions/rmagic.py
index 21aeeab..753e0db 100644
--- a/IPython/extensions/rmagic.py
+++ b/IPython/extensions/rmagic.py
@@ -337,10 +337,14 @@ class RMagics(Magics):
         help='Convert these objects to data.frames and return as structured arrays.'
         )
     @argument(
-        '-u', '--units', type=int,
+        '-u', '--units', type=unicode, choices=["px", "in", "cm", "mm"],
         help='Units of png plotting device sent as an argument to *png* in R. One of ["px", "in", "cm", "mm"].'
         )
     @argument(
+        '-r', '--res', type=int,
+        help='Resolution of png plotting device sent as an argument to *png* in R. Defaults to 72 if *units* is one of ["in", "cm", "mm"].'
+        )
+    @argument(
         '-p', '--pointsize', type=int,
         help='Pointsize of png plotting device sent as an argument to *png* in R.'
         )
@@ -524,7 +528,12 @@ class RMagics(Magics):
                         raise NameError("name '%s' is not defined" % input)
                 self.r.assign(input, self.pyconverter(val))
 
-        png_argdict = dict([(n, getattr(args, n)) for n in ['units', 'height', 'width', 'bg', 'pointsize']])
+        if getattr(args, 'units') is not None:
+            if args.units != "px" and getattr(args, 'res') is None:
+                args.res = 72
+            args.units = '"%s"' % args.units
+
+        png_argdict = dict([(n, getattr(args, n)) for n in ['units', 'res', 'height', 'width', 'bg', 'pointsize']])
         png_args = ','.join(['%s=%s' % (o,v) for o, v in png_argdict.items() if v is not None])
         # execute the R code in a temporary directory
 
diff --git a/IPython/extensions/tests/test_rmagic.py b/IPython/extensions/tests/test_rmagic.py
index 4e128a8..43be445 100644
--- a/IPython/extensions/tests/test_rmagic.py
+++ b/IPython/extensions/tests/test_rmagic.py
@@ -71,7 +71,7 @@ def test_cell_magic():
     r = resid(a)
     xc = coef(a)
     '''
-    ip.run_cell_magic('R', '-i x,y -o r,xc a=lm(y~x)', snippet)
+    ip.run_cell_magic('R', '-i x,y -o r,xc -w 150 -u mm a=lm(y~x)', snippet)
     np.testing.assert_almost_equal(ip.user_ns['xc'], [3.2, 0.9])
     np.testing.assert_almost_equal(ip.user_ns['r'], np.array([-0.2,  0.9, -1. ,  0.1,  0.2]))