FLAG_max_new_space_size is in MB.

src/api.cc

 // 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.
 
 #include "api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #include <cmath>  // For isnan.
 #include "../include/v8-debug.h"
 #include "../include/v8-profiler.h"
@@ skipping 456 matching lines @@
     set_code_range_size(
         i::Min(512 * i::MB, static_cast<int>(virtual_memory_limit >> 3)));
   }
 }
 
 
 bool SetResourceConstraints(Isolate* v8_isolate,
                             ResourceConstraints* constraints) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   int new_space_size = constraints->max_new_space_size();
-  int old_gen_size = constraints->max_old_space_size();
+  int old_space_size = constraints->max_old_space_size();
   int max_executable_size = constraints->max_executable_size();
   int code_range_size = constraints->code_range_size();
-  if (new_space_size != 0 || old_gen_size != 0 || max_executable_size != 0 ||
+  if (new_space_size != 0 || old_space_size != 0 || max_executable_size != 0 ||
       code_range_size != 0) {
     // After initialization it's too late to change Heap constraints.
     ASSERT(!isolate->IsInitialized());
     bool result = isolate->heap()->ConfigureHeap(new_space_size / 2,
-                                                 old_gen_size,
+                                                 old_space_size,
                                                  max_executable_size,
                                                  code_range_size);
     if (!result) return false;
   }
   if (constraints->stack_limit() != NULL) {
     uintptr_t limit = reinterpret_cast<uintptr_t>(constraints->stack_limit());
     isolate->stack_guard()->SetStackLimit(limit);
   }
 
   isolate->set_max_available_threads(constraints->max_available_threads());
@@ skipping 7098 matching lines @@
   Isolate* isolate = reinterpret_cast<Isolate*>(info.GetIsolate());
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 } }  // namespace v8::internal