my $scalewidth  = $Main->ScaleWidth();
my $scaleheight = $Main->ScaleHeight();
my $Main_msgs = $Main->AddTextfield(
    ...
    -size => [$scalewidth, $scaleheight],
    ...
);

##</code><code>##

# The sub is named for the control name and event name
sub Main_Resize {
    my $scalewidth  = $Main->ScaleWidth();
    my $scaleheight = $Main->ScaleHeight();
    $Main_msgs->Resize($scalewidth, $scaleheight);
    return 1;
}