You can adjust the size of the listbox by configuring its -height and -width.

You can also allow the geometry manager to control a widget's size. For pack, you set -fill and -expand options when you pack the widget. For grid, you set the -sticky option to cause the widget to grow to fill the grid cell. You can also use gridRowconfigure and gridColumnconfigure with the -weight option to make a row/column expand as a window is resized.

Here's a snippet you can paste in to replace line 38 of the code above:

$box1->grid( $box2, -sticky => 'ns' );
$mw->gridRowconfigure(2, -weight => 1 );
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It's worth the time to really dig into and understand the pack and grid geometry managers. I've played with the other managers, but have never needed anything else for a real project.