Which version of PAR/OLE do you have? I can't get your example to work, but I've no problem pp-ing up the following with the latest PAR/Win32::OLE (PAR 0.86, Win32::OLE 0.17).

use strict;
use Win32::OLE qw[in with];
my $fs = Win32::OLE->CreateObject('Scripting.FileSystemObject');
my $size = 0;
my $d = $fs->GetFolder("$ARGV[0]");
eval
{ 
    $size = $d->size( );    
    my $size_gb = ($size/1024/1024);
    print "Size: ".$size_gb."Mb ($size byte)";        
};
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Works without a hitch. My perl is ActivePerl 5.8.4.