I use the below for a Pane widget which is to hold a Label widget which is to hold a multi-line string for display in a pop-up window.

# Calculate dimensions of message for pane, etc. Then
# configure widget to that height and width.
sub size_widget_for_string {
    my ($wgt, $msg, $font) = @_;

    # Measure widest line of string for given font.
    my @msg = split "\n", $msg;
    my $hght = 1.5 * $wgt->fontMetrics($font, -linespace);
    my $wdth = 0;
    foreach (@msg) {
        my $line_wdth = $wgt->fontMeasure($font, " $_ ");
        $wdth = $line_wdth if $wdth < $line_wdth;
        $hght += $wgt->fontMetrics($font, -linespace);
    }

    # Limit size to reasonable maximums.
    my $max_hght = 0.6 * $wgt->screenheight;
    my $max_wdth  = 0.9 * $wgt->screenwidth;
    $hght = $max_hght if $hght > $max_hght;
    $wdth = $max_wdth if $wdth > $max_wdth;

    # Configure the widget.
    $wgt->configure( -width => $wdth, -height => $hght );
}
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