It won't give you the sizes directly, but the easiest way to convince yourself that your debug code is being exclused is to use B::Deparse, which for reasons I do not understand is invoked by adding -MO=Deparse to your command line.

This is what the output looks like when debug is enabled:

C:\test>type junk.pl
#! perl -slw
use strict;
use constant DEBUG => 1;

if( DEBUG ) {
    print "Debugging...";
}

print "Not debugging...";


C:\test>junk.pl
Debugging...
Not debugging...

C:\test>perl -MO=Deparse junk.pl
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant ('DEBUG', 1);
use strict 'refs';
do {
    print 'Debugging...'
};
print 'Not debugging...';
junk.pl syntax OK
[download]

Note how the conditional block has been replaced by a do block.

And this is what it looks like when debug is disabled:

C:\test>type junk.pl
#! perl -slw
use strict;
use constant DEBUG => 0;

if( DEBUG ) {
    print "Debugging...";
}

print "Not debugging...";


C:\test>junk.pl
Not debugging...

C:\test>perl -MO=Deparse junk.pl
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use constant ('DEBUG', 0);
use strict 'refs';
'???';
print 'Not debugging...';
junk.pl syntax OK
[download]

Note that the entire conditional block has been replaced by '???'.