Well, you can set it that way:

my $MW = MainWindow->new( -height => 600, -width => 600 );
[download]

or

my $MW = MainWindow->new;
$MW->geometry( "600x600" );
[download]

but if you arrange items that don't fit into that size, your window will enlarge

in perldoc Tk::Wm the methods ->maxsize and ->minsize could be interesting for you