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;
[download]

Rosetta Code mentions the zoomed state as a way of maximizing a window, it doesn't work on other platforms, though. It also doesn't provide a way to capture the event.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

#!/usr/bin/perl
use warnings;
use strict;

use Tk;

my $top = MainWindow->new;

$top->geometry('200x200+200+200');

$top->overrideredirect(1);

$top->Button(
    -text    => 'Maximize',
    -command => sub { $top->geometry($top->screenwidth . 'x' . $top->s
+creenheight . '+0+0');

             print "Maximized\n";
                  })->pack;


$top->Button(
    -text    => 'Exit',
    -command => sub { $top->destroy }
)->pack;

MainLoop;
[download]