FLAG_max_new_space_size is in MB.

src/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_H_
 #define V8_HEAP_H_
 
 #include <cmath>
 
 #include "allocation.h"
@@ skipping 531 matching lines @@
   DONT_INITIALIZE_ARRAY_ELEMENTS,
   INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE
 };
 
 
 class Heap {
  public:
   // Configure heap size before setup. Return false if the heap has been
   // set up already.
   bool ConfigureHeap(int max_semispace_size,
-                     intptr_t max_old_gen_size,
+                     intptr_t max_old_space_size,
                      intptr_t max_executable_size,
                      intptr_t code_range_size);
   bool ConfigureHeapDefault();
 
   // Prepares the heap, setting up memory areas that are needed in the isolate
   // without actually creating any objects.
   bool SetUp();
 
   // Bootstraps the object heap with the core set of objects required to run.
   // Returns whether it succeeded.
@@ skipping 2244 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_