Buttons and Labels in Tk are text-line oriented. You can't really set the height or width in pixels, when you set the width, it's the character width of the text (at the font it's using). The height can only be changed by putting newlines in it's text. Here are 2 examples:

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

my $mw = new MainWindow();

my $frame = $mw->Frame(-bd => 2, -relief => 'raised')->pack(-expand =>
+ 1, -fill => 'both');

my $scrolledMListbox = $frame->Scrolled(
        qw/MListbox -selectmode browse -scrollbars oe/
        )->pack(-expand => 1, -fill => 'both');

my @list = ( "a", "b" );

$scrolledMListbox->columnInsert('end',  -text => "\nPath\n");
$scrolledMListbox->columnInsert('end', -text => "\nModified\n");
$scrolledMListbox->columnInsert('end',  -text => "\nDir\n");
$scrolledMListbox->columnInsert('end',  -text => "\nFile\n");

$scrolledMListbox->insert('end', [ "a", "b", "s", "l"] );
$scrolledMListbox->insert('end', [ "c", "d", "f", "e"] );

MainLoop;
[download]

or you can try to set a font size.

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

my $mw = new MainWindow();

$mw->fontCreate('big', -family=>'arial',
   -weight=>'bold', -size=> 18 );

$mw->fontCreate('medium', -family=>'arial',
   -weight=>'bold', -size=> 14 );


my $frame = $mw->Frame(-bd => 2, -relief => 'raised')->pack(-expand =>
+ 1, -fill => 'both');

my $scrolledMListbox = $frame->Scrolled(
        qw/MListbox -selectmode browse -scrollbars oe -font medium /
        )->pack(-expand => 1, -fill => 'both');

my @list = ( "a", "b" );

$scrolledMListbox->columnInsert('end',  -text => "Path",-font => 'big'
+);
$scrolledMListbox->columnInsert('end', -text => "Modified",-font => 'b
+ig');
$scrolledMListbox->columnInsert('end',  -text => "Dir",-font => 'big')
+;
$scrolledMListbox->columnInsert('end',  -text => "File",-font => 'big'
+);

$scrolledMListbox->insert('end', [ "a", "b", "s", "l"]  );
$scrolledMListbox->insert('end', [ "c", "d", "f", "e"]  );

MainLoop;
[download]