Fixed is a bitmapped font and only supports a few predetermined sizes. It will select the size that is closest to the requested size. The largest size supported seems to be about 70. On my (WinXP) system, once you get above 121, it turns to Helvetica.

Here's a little utility I wrote to mess around with fonts and sizes. Use the up and down arrows to change the size quickly.

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

my $fontname = 'Times';
my $fontsize = 200;

my $top = MainWindow->new;
my $cframe = $top->Frame->pack(-anchor => 'w');
my $label = $top->Label(
    -text => 'This is a label',
    -font => "{$fontname} $fontsize",
)->pack;

my $sizelabel = $cframe->Label(
    -text => $fontsize
)->grid(-row => 1, -column => 3, -padx => 2);

my $fontlist = $cframe->BrowseEntry(
    -label     => 'Font',
    -browsecmd => sub{ fontconfig($label, $fontname, $fontsize) },
    -variable  => \$fontname,
)->grid(-row => 1, -column => 1, -padx => 8);

$fontlist->insert ('end', sort $top->fontFamilies);

my $smaller = $cframe->Button(
    -text    => 'Smaller',
    -command => sub{
                        $fontsize--;
                        fontconfig($label, $fontname, $fontsize);
                        $sizelabel->configure(-text => $fontsize);
                    },
)->grid(-row => 1, -column => 2, -padx => 2);

my $bigger = $cframe->Button(
    -text    => 'Bigger',
    -command => sub{
                        $fontsize++;
                        fontconfig($label, $fontname, $fontsize);
                        $sizelabel->configure(-text => $fontsize);
                    },
)->grid(-row => 1, -column => 4, -padx => 2);

$top->bind('<Up>' => sub{$bigger->invoke});
$top->bind('<Down>' => sub{$smaller->invoke});
$top->focus;

MainLoop;

sub fontconfig{
    my ($widget, $fontname, $fontsize) = @_;
    $widget->configure(-font => "{$fontname} $fontsize");
}

__END__
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