Like this?:

...
my $height = $mw->screenheight;
my $width  = $mw->screenwidth;

my $geom = $mw->geometry;

$mw->title("New Title ($geom)");
[download]

Update:

Changed code to show geometry of the window instead of the screen.

Update2: Width and height (for any widget):

my $height = $mw->height;
my $width  = $mw->width;

$mw->title("New Title ($width x $height)");
[download]

Other options are here: Tk::Widget