add_action('admin_head', 'custom_post_list_column_width');
function custom_post_list_column_width() {
    echo '<style type="text/css">';
    // Adjust the Title column width
    echo '.table-view-list.posts .column-title { width: 350px !important; }';
    // Adjust the Categories column width
    echo '.table-view-list.posts .column-categories { width: 150px !important; }';
    echo '</style>';
}
Use code with caution.
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