FLAG_max_new_space_size is in MB.

src/flag-definitions.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.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 451 matching lines @@
 
 // frames.cc
 DEFINE_int(max_stack_trace_source_length, 300,
            "maximum length of function source code printed in a stack trace.")
 
 // full-codegen.cc
 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")
 DEFINE_bool(trace_gc, false,
             "print one trace line following each garbage collection")
 DEFINE_bool(trace_gc_nvp, false,
             "print one detailed trace line in name=value format "
             "after each garbage collection")
 DEFINE_bool(trace_gc_ignore_scavenger, false,
             "do not print trace line after scavenger collection")
@@ skipping 420 matching lines @@
 #undef DEFINE_ALIAS_float
 #undef DEFINE_ALIAS_args
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA