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;