You can get the new size when it is changed by binding to Configure.

Here's an example that does that:

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1213422

use strict;
use warnings;
use Tk;

my $mw = MainWindow->new;
my $c = $mw->Canvas( -width => 400, -height => 400,
  -bg => 'red',
  -highlightthickness => 0,
  )->pack;
for my $row (0..7)
  {
  my $color = $row % 2;
  for my $col (0..7)
    {
    $c->createRectangle($row*50, $col*50, $row*50+50, $col*50+50,
      -fill => qw( gold2 green)[$color++ % 2],
      -outline => undef,
      );
    }
  }

$mw->bind('<Configure>' => sub
  {
  print "width ", $mw->width, " height ", $mw->height, "\n";
  } );

MainLoop;
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