See Re: Tk: Set text size in a widget and "perldoc Tk::Font" and "perldoc Tk::X11Font", but using the Canvas is always a good bet.

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

my $mw = new MainWindow;

$mw->fontCreate('big',
    -family=>'arial',
    -weight=>'bold',
    -size=> 28
     );

my $c = $mw->Canvas(-bg=>'white')->pack;

#---determine font spacing by making a capital W---
   my $fonttest =  $c->createText(100,100,
              -fill    => 'black',
              -text    => 'W',
              -font => 'big'
              );

    my ($bx,$by,$bx1,$by1) = $c->bbox($fonttest);
    $c->{'f_width'} = $bx1 - $bx;
    $c->{'f_height'} = $by1 - $by;
    
    print 'width ',$c->{'f_width'},'   ',$c->{'f_height'},"\n";
     
MainLoop;
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