FLAG_max_new_space_size is in MB.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index 47d87eaa50a044426cb446bb451c49d7bed6d3e0..32bd20e1310b4cce410bab966dd9d5fa6ac2f9cb 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -469,8 +469,10 @@ DEFINE_bool(always_inline_smi_code, false,
             "always inline smi code in non-opt code")
 
 // heap.cc
-DEFINE_int(max_new_space_size, 0, "max size of the new generation (in kBytes)")
-DEFINE_int(max_old_space_size, 0, "max size of the old generation (in Mbytes)")
+DEFINE_int(max_new_space_size, 0,
+    "max size of the new space consisting of two semi-spaces which are half"
+    "the size (in MBytes)")
+DEFINE_int(max_old_space_size, 0, "max size of the old space (in Mbytes)")
 DEFINE_int(max_executable_size, 0, "max size of executable memory (in Mbytes)")
 DEFINE_bool(gc_global, false, "always perform global GCs")
 DEFINE_int(gc_interval, -1, "garbage collect after <n> allocations")