See Tk::WidgetDump for figuring out widgets, slightly easier than reading the source (of Tk::FileSelect)

If you don't want all children to have same font size, select the child you want, use $fs->Subwidget('dir_entry')...

#!/usr/bin/perl --
use strict;
use warnings;
use Tk;
use Tk::WidgetDump;
my $mw = Tk::MainWindow->new;
my $fs = $mw->FileSelect( -font => "Arial 24" );
FontForKids( $mw , "Arial 24" );
$mw->WidgetDump; 
$fs->Show;
sub FontForKids {
    my( $mw , $font ) = @_;
    my @kids = $mw->children ;
    while( @kids ){
        my $kid = shift @kids;
        push @kids, $kid->children;
        eval { $kid->configure( -font => $font ); 1 }
##        or warn $@;
    }
    return $mw;
}
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